Peptides and compounds that bind to the thrombopoietin receptor

ABSTRACT

Described are peptides and peptide mimetics that bind to and activate the thrombopoietin receptor. Such peptides and peptide mimetics are useful in methods for treating hematological disorders and particularly, thrombocytopenia resulting from chemotherapy, radiation therapy, or bone marrow transfusions as well as in diagnostic methods employing labeled peptides and peptide mimetics.

BACKGROUND OF THE INVENTION

The present invention provides peptides and compounds that bind to and activate the thrombopoietin receptor (c-mp1 or TPO-R) or otherwise act as a TPO agonist. The invention has application in the fields of biochemistry and medicinal chemistry and particularly provides TPO agonists for use in the treatment of human disease.

Megakaryocytes are bone marrow-derived cells, which are responsible for producing circulating blood platelets. Although comprising <0.25% of the bone marrow cells in most species, they have >10 times the volume of typical marrow cells. See Kuter et al. (1994) Proc. Natl. Acad. Sci. USA 91:11104-11108. Megakaryocytes undergo a process known as endomitosis whereby they replicate their nuclei but fail to undergo cell division and thereby give rise to polyploid cells. In response to a decreased platelet count, the endomitotic rate increases, higher ploidy megakaryocytes are formed, and the number of megakaryocytes may increase up to 3-fold. See Harker (1968) J. Clin. Invest. 47:458-465. In contrast, in response to an elevated platelet count, the endomitotic rate decreases, lower ploidy megakaryocytes are formed, and the number of megakaryocytes may decrease by 50%.

The exact physiological feedback mechanism by which the mass of circulating platelets regulates the endomitotic rate and number of bone marrow megakaryocytes is not known. The circulating thrombopoietic factor involved in mediating this feedback loop is now thought to be thrombopoietin (TPO). More specifically, TPO has been shown to be the main humoral regulator in situations involving thrombocytopenia. See, e.g., Metcalf (1994) Nature 369:519-520. TPO has been shown in several studies to increase platelet counts, increase platelet size, and increase isotope incorporation into platelets of recipient animals. Specifically, TPO is thought to affect megakaryocytopoiesis in several ways: (1) it produces increases in megakaryocyte size and number; (2) it produces an increase in DNA content, in the form of polyploidy, in megakaryocytes; (3) it increases megakaryocyte endomitosis; (4) it produces increased maturation of megakaryocytes; and (5) it produces an increase in the percentage of precursor cells, in the form of small acetylcholinesterase-positive cells, in the bone marrow.

Because platelets (thrombocytes) are necessary for blood clotting and when their numbers are very low a patient is at serious risk of death from catastrophic hemorrhage, TPO has potential useful application in both the diagnosis and the treatment of various hematological disorders, for example, diseases primarily due to platelet defects. In addition, recent studies have provided a basis for the projection of efficacy of TPO therapy in the treatment of thrombocytopenia, and particularly thrombocytopenia resulting from chemotherapy, radiation therapy, or bone marrow transplantation as treatment for cancer or lymphoma. See, e.g., McDonald (1992) Am. J. Ped. Hematology/Oncology 14:8-21.

The gene encoding TPO has been cloned and characterized. See Kuter et al. (1994) Proc. Natl. Acad. Sci. USA 91:11104-11108; Barley et al. (1994) Cell 77:1117-1124; Kaushansky et al. (1994) Nature 369:568-571; Wendling et al. (1994) Nature 369:571-574; and Sauvage et al. (1994) Nature 369:533-538. Thrombopoietin is a glycoprotein with at least two forms, with apparent molecular masses of 25 kDa and 31 kDa, with a common N-terminal amino acid sequence. See, Bartley et al. (1994) Cell 77:1117-1124. Thrombopoietin appears to have two distinct regions separated by a potential Arg--Arg cleavage site. The amino-terminal region is highly conserved in man and mouse, and has some homology with erythropoietin and interferon-a and interferon-b. The carboxy-terminal region shows wide species divergence.

The DNA sequences and encoded peptide sequences for human TPO-R (also known as c-mpl) have been described. See Vigon et al. (1992) Proc. Natl. Acad. Sci. USA 89:5640-5644. TPO-R is a member of the haematopoietin growth factor receptor family, a family characterized by a common structural design of the extracellular domain, including four conserved cysteine residues in the N-terminal portion and a WSXWS motif close to the transmembrane region. See Bazan (1990) Proc. Natl. Acad. Sci. USA 87:6934-6938. Evidence that this receptor plays a functional role in hematopoiesis includes observations that its expression is restricted to spleen, bone marrow, or fetal liver in mice (see Souyri et al. (1990) Cell 63:1137-1147) and to megakaryocytes, platelets, and CD34⁺ cells in humans (see Methia et al. (1993) Blood 82:1395-1401). Furthermore, exposure of CD34⁺ cells to synthetic oligonucleotides antisense to mpl RNA significantly inhibits the appearance of megakaryocyte colonies without affecting erythroid or myeloid colony formation. Some workers postulate that the receptor functions as a homodimer, similar to the situation with the receptors for G-CSF and erythropoietin.

The availability of cloned genes for TPO-R facilitates the search for agonists of this important receptor. The availability of the recombinant receptor protein allows the study of receptor-ligand interaction in a variety of random and semi-random peptide diversity generation systems. These systems include the "peptides on plasmids" system described in U.S. Pat. Nos. 5,270,170 and 5,338,665; the "peptides on phage" system described in U.S. patent application Ser. No. 07/718,577, filed Jun. 20, 1991, U.S. patent application Ser. No. 07/541,108, filed Jun. 20, 1990, and in Cwirla et al., (1990) Proc. Natl. Acad. Sci. USA 87:6378-6382; the "encoded synthetic library" system described in U.S. patent application Ser. Nos. 08/146,886, filed Nov. 12, 1993, 07/946,239, filed Sep. 16, 1992, and 07/762,522, filed Sep. 18, 1991; and the "very large scale immobilized polymer synthesis" system described in U.S. Pat. No. 5,143,854; PCT patent publication Ser. No. 90/15070, published Dec. 13, 1990; U.S. patent application Ser. No. 624,120, filed Dec. 6, 1990; Fodor et al., Feb. 15, 1991, Science 251:767-773; Dower and Fodor (1991) Ann. Rep. Med. Chem. 26:271-180; and U.S. patent application Ser. No. 805,727, filed Dec. 6, 1991; each of the foregoing patent applications and publications is incorporated herein by reference.

The slow recovery of platelet levels in patients suffering from thrombocytopenia is a serious problem, and has lent urgency to the search for a blood growth factor agonist able to accelerate platelet regeneration. The present invention provides such an agonist.

SUMMARY OF THE INVENTION

This invention is directed, in part, to the novel and unexpected discovery that defined low molecular weight peptides and peptide mimetics have strong binding properties to the TPO-R and can activate the TPO-R. Accordingly, such peptides and peptide mimetics are useful for therapeutic purposes in treating conditions mediated by TPO (e.g., thrombocytopenia resulting from chemotherapy, radiation therapy, or bone marrow transfusions) as well as for diagnostic purposes in studying the mechanism of hematopoiesis and for the in vitro expansion of megakaroycytes and committed progenitor cells.

Peptides and peptide mimetics suitable for therapeutic and/or diagnostic purposes have an IC₅₀ of about 2 mM or less, as determined by the binding affinity assay set forth in Example 3 below wherein a lower IC₅₀ correlates to a stronger binding affinity to TPO-R. For pharmaceutical purposes, the peptides and peptidomimetics preferably have an IC₅₀ of no more than about 100 mm. In a preferred embodiment, the molecular weight of the peptide or peptide mimetic is from about 250 to about 5000 daltons.

When used for diagnostic purposes, the peptides and peptide mimetics preferably are labeled with a detectable label and, accordingly, the peptides and peptide mimetics without such a label serve as intermediates in the preparation of labeled peptides and peptide mimetics.

Peptides meeting the defined criteria for molecular weight and binding affinity for TPO-R comprise 9 or more amino acids wherein the amino acids are naturally occurring or synthetic (non-naturally occurring) amino acids. Peptide mimetics include peptides having one or more of the following modifications:

peptides wherein one or more of the peptidyl --C(O)NR--! linkages (bonds) have been replaced by a non-peptidyl linkage such as a --CH₂ -carbamate linkage --CH₂ --OC(O)NR--!; a phosphonate linkage; a --CH₂ -sulfonamide --CH₂ --S(O)₂ NR--! linkage; a urea --NHC(O)NH--! linkage; a --CH₂ -secondary amine linkage; or an alkylated peptidyl linkage --C(O)NR⁶ -- where R⁶ is lower alkyl!;

peptides wherein the N-terminus is derivatized to a --NRR¹ group; to a --NRC(O)R group; to a --NRC(O)OR group; to a --NRS(O)₂ R group; to a --NHC(O)NHR group where R and R¹ are hydrogen or lower alkyl with the proviso that R and R¹ are not both hydrogen; to a succinimide group; to a benzyloxycarbonyl-NH--(CBZ--NH--) group; or to a benzyloxycarbonyl-NE-- group having from 1 to 3 substituents on the phenyl ring selected from the group consisting of lower alkyl, lower alkoxy, chloro, and bromo; or

peptides wherein the C terminus is derivatized to --C(O)R² where R² is selected from the group consisting of lower alkoxy, and --NR³ R⁴ where R³ and R⁴ are independently selected from the group consisting of hydrogen and lower alkyl.

Accordingly, preferred peptides and peptide mimetics comprise a compound having:

(1) a molecular weight of less than about 5000 daltons, and

(2) a binding affinity to TPO-R as expressed by an IC₅₀ of no more than about 100 mm,

wherein from zero to all of the --C(O)NH-- linkages of the peptide have been replaced by a linkage selected from the group consisting of a --CR₂ OC(O)NR-- linkage; a phosphonate linkage; a --CH₂ S(O)₂ NR-- linkage; a --CH₂ NR-- linkage; and a --C(O)NR⁶ -- age; and a --NHC(O)NH-- linkage where R is hydrogen or lower alkyl and R⁶ is lower alkyl,

further wherein the N-terminus of said peptide or peptide mimetic is selected from the group consisting of a --NRR¹ group; a --NRC(O)R group; a --NRC(O)OR group; a --NRS(O)₂ R group; a --NHC(O)NHR group; a succinimide group; a benzyloxycarbonyl-NH-- group; and a benzyloxycarbonyl-NH-- group having from 1 to 3 substituents on the phenyl ring selected from the group consisting of lower alkyl, lower alkoxy, chloro, and bromo, where R and R¹ are independently selected from the group consisting of hydrogen and lower alkyl,

and still further wherein the C-terminus of said peptide or peptide mimetic has the formula --C(O)R² where R² is selected from the group consisting of hydroxy, lower alkoxy, and --NR³ R⁴ where R³ and R⁴ are independently selected from the group consisting of hydrogen and lower alkyl and where the nitrogen atom of the --NR³ R⁴ group can optionally be the amine group of the N-terminus of the peptide so as to form a cyclic peptide,

and physiologically acceptable salts thereof.

In a related embodiment, the invention is directed to a labeled peptide or peptide mimetic comprising a peptide or peptide mimetic described as above having covalently attached thereto a label capable of detection.

Preferred peptides for use in this invention include peptides having a core structure comprising sequence of amino acids:

    V R X.sub.1 Q X.sub.2 X X X X.sub.3 (SEQ ID NO.1)

where X₁ is D or E; X₂ is I or V; X₃ is F, H, L, M, T, V, W, or Y; and each X residue is independently selected from any of the 20 genetically coded L-amino acids, their stereoisomeric D-amino acids; and non-natural amino acids. Preferably, each X residue is independently selected from any of the 20 genetically coded L-amino acids and their stereoisomeric D-amino acids. More preferably, each X residue is independently selected from any of the 20 genetically coded L-amino acids; and X₃ is F, L, W, or Y.

In a preferred embodiment, the core peptide comprises a sequence of amino acids:

    V R X.sub.1 Q X.sub.2 X.sub.4 X.sub.5 X.sub.6 X.sub.3 (SEQ ID NO.2)

where X₄ is A, C, F, H, I, L, M, Q, S, T, V, W, or Y; X₅ is A, C, G, I, K, L, M, Q, R, S, W, or V; and X₆ is A, C, F, H, L, R, S, W, or Y. More preferably, X₄ is A, C, F, I, L, M, S, W, or Y; X₅ is A, L, M, or W; and X₆ is F, H, R, W, or Y.

In a more preferred embodiment, the core peptide comprises a sequence of amino acids:

    X X V R X.sub.1 Q X.sub.2 X.sub.4 X.sub.5 X.sub.6 X.sub.3 (SEQ ID NO.3)

where each X residue is independently selected from any of the 20 genetically coded L-amino acids, their stereoisomeric D-amino acids; and non-natural amino acids. Preferably, each X residue is independently selected from any of the 20 genetically coded L-amino acids and their stereoisomeric D-amino acids.

More preferably, the core peptide comprises a sequence of amino acids:

    X.sub.7 X.sub.8 V R X.sub.1 Q X.sub.2 X.sub.4 X.sub.5 X.sub.6 X.sub.3 (SEQ ID NO.4)

where X₇ is A, E, G, I, L, N, Q, P, R, S, T, V, or Y; and X₈ is D, E, G, K, R, S, T, or Y. Even more preferably, X₇ is E, G L, R, S, or T; and X₈ is E, G, R, or S.

Particularly preferred peptides include: Q D P R G V R E Q V I A R L C (SEQ ID NO.5) and its disulfide linked homodimer:

    QDPRGVREQVIARLC                   |     QDPRGVREQVIARLC

The compounds described herein are useful for the prevention and treatment of diseases mediated by TPO, and particularly for treating hematological disorders, including but not limited to, thrombocytopenia resulting from chemotherapy, radiation therapy, or bone marrow transfusions. Thus, the present invention also provides a method for treating wherein a patient having a disorder that is susceptible to treatment with a TPO agonist receives, or is administered, a therapeutically effective dose or amount of a compound of the present invention.

The invention also provides for pharmaceutical compositions comprising one or more of the compounds described herein and a physiologically acceptable carrier. These pharmaceutical compositions can be in a variety of forms including oral dosage forms, as well as inhalable powders and solutions and injectable and infusible solutions.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a graphical depiction of the results of the TPO-R transfected Ba/F3 cell proliferation assay, as described in Example 2, for the results for the homodimer:

    QDPRGVREQVIARLC                   |     QDPRGVREQVIARLC

(AF 12333--TPO-R). The results with the parental cell line are also shown (AF 12333--parental).

FIGS. 2A-C illustrates the construction of peptides-on-plasmids libraries in vector pJS142. FIG. 2A shows a restriction map and position of the genes. The library plasmid includes the rrnB transcriptional terminator, the bla gene to permit selection on ampicillin, the M13 phage intragenic region (M13 IG) to permit rescue of single-stranded DNA, a plasmid replication origin (ori), two lacO_(s) sequence, and the araC gene to permit positive and negative regulation of the araB promoter driving expression of the lac fusion gene. FIG. 2B shows the sequence of the cloning region at the 3' end of the lac I gene, including the SfiI and EagI sites used during library construction. FIG. 2C shows the ligation of annealed library oligonucleotides, ON-829 and ON-830, to SfiI sites of pJS142 to produce a library. Singles spaces in the sequence indicate sites of ligation.

FIGS. 3A-B illustrate cloning into the pELM3 and pELM15 MBP vectors. FIG. 3A shows the sequence at the 3' end of the malE fusion gene, including the MBP coding sequence, the poly asparagine linker, the factor Xa protease cleavagge site, and the available cloning sites. The remaining portions of the vectors are derived from pMALc2 (pELM3) and pMALp2 (pELM15), available from New England Biolabs. FIG. 3B shows the sequence of the vectors after transfer of the BspEII-ScaI library fragment into AgeI-ScaI digested pELM3/pELM15. The transferred sequence includes the sequence encoding the GGG peptide linker from the pJS142 library.

FIG. 4 A depicts a restriction map and position of the genes for the construction of headpiece diemr libraries in vector pCMG14. The library plasmid includes: the rrnB transcriptional terminator, the bla gene to permit selection on ampicillin, the M13 phage intragenic region (M13 IG) to permit rescue of single-stranded DNA, a plasmid replication origin (ori), one lacO_(s) sequence, and the araC gene to permit positive and negative regulation of the araB promoter driving expression of the headpiece dimer fusion gene. FIG. 4B depicts the sequence of the cloning region at the 3' end of the headpiece dimer gene, including the SfiI and EagI sites used during library construction. FIG. 4 C shows the ligation of annealed ON-1679, ON-829, and ON-830 to SfiI sites of pCMG14 to produce a library. Singles spaces in the sequence indicate sites of ligation.

DESCRIPTION OF SPECIFIC EMBODIMENTS

I. Definitions and General Parameters

The following definitions are set forth to illustrate and define the meaning and scope of the various terms used to describe the invention herein.

"Agonist" refers to a biologically active ligand which binds to its complementary biologically active receptor and activates the latter either to cause a biological response in the receptor or to enhance preexisting biological activity of the receptor.

"Pharmaceutically acceptable salts" refer to the non-toxic alkali metal, alkaline earth metal, and ammonium salts commonly used in the pharmaceutical industry including the sodium, potassium, lithium, calcium, magnesium, barium, ammonium, and protamine zinc salts, which are prepared by methods well known in the art. The term also includes non-toxic acid addition salts, which are generally prepared by reacting the compounds of this invention with a suitable organic or inorganic acid. Representative salts include the hydrochloride, hydrobromide, sulfate, bisulfate, acetate, oxalate, valerate, oleate, laurate, borate, benzoate, lactate, phosphate, tosylate, citrate, maleate, fumarate, succinate, tartrate, napsylate, and the like.

"Pharmaceutically acceptable acid addition salt" refers to those salts which retain the biological effectiveness and properties of the free bases and which are not biologically or otherwise undesirable, formed with inorganic acids such as hydrochloric acid, hydrobromic acid, sulfuric acid, nitric acid, phosphoric acid and the like, and organic acids such as acetic acid, propionic acid, glycolic acid, pyruvic acid, oxalic acid, malic acid, malonic acid, succinic acid, maleic acid, fumaric acid, tartaric acid, citric acid, benzoic acid, cinnamic acid, mandelic acid, menthanesulfonic acid, ethanesulfonic acid, p-toluenesulfonic acid, salicylic acid and the like. For a description of pharmaceutically acceptable acid addition salts as prodrugs, see Bundgaard, H., supra.

"Pharmaceutically acceptable ester" refers to those esters which retain, upon hydrolysis of the ester bond, the biological effectiveness and properties of the carboxylic acid or alcohol and are not biologically or otherwise undesirable. For a description of pharmaceutically acceptable esters as prodrugs, see Bundgaard, E., ed., (1985) Design of Prodrugs, Elsevier Science Publishers, Amsterdam. These esters are typically formed from the corresponding carboxylic acid and an alcohol. Generally, ester formation can be accomplished via conventional synthetic techniques. (See, e.g., March Advanced Organic Chemistry, 3rd Ed., John Wiley & Sons, New York (1985) p. 1157 and references cited therein, and Mark et al. Encyclopedia of Chemical Technology, John Wiley & Sons, New York (1980).) The alcohol component of the ester will generally comprise (i) a C₂ -C₁₂ aliphatic alcohol that can or can not contain one or more double bonds and can or can not contain branched carbons or (ii) a C₇ -C₁₂ aromatic or heteroaromatic alcohols. This invention also contemplates the use of those compositions which are both esters as described herein and at the same time are the pharmaceutically acceptable acid addition salts thereof.

"Pharmaceutically acceptable amide" refers to those amides which retain, upon hydrolysis of the amide bond, the biological effectiveness and properties of the carboxylic acid or amine and are not biologically or otherwise undesirable. For a description of pharmaceutically acceptable amides as prodrugs, see Bundgaard, H., ed., (1985) Design of Prodrugs, Elsevier Science Publishers, Amsterdam. These amides are typically formed from the corresponding carboxylic acid and an amine. Generally, amide formation can be accomplished via conventional synthetic techniques. (See, e.g., March Advanced Organic Chemistry, 3rd Ed., John Wiley & Sons, New York (1985) p. 1152 and Mark et al. Encyclopedia of Chemical Technology, John Wiley & Sons, New York (1980).) This invention also contemplates the use of those compositions which are both amides as described herein and at the same time are the pharmaceutically acceptable acid addition salts thereof.

"Pharmaceutically or therapeutically acceptable carrier" refers to a carrier medium which does not interfere with the effectiveness of the biological activity of the active ingredients and which is not toxic to the host or patient.

"Stereoisomer" refers to a chemical compound having the same molecular weight, chemical composition, and constitution as another, but with the atoms grouped differently. That is, certain identical chemical moieties are at different orientations in space and, therefore, when pure, has the ability to rotate the plane of polarized light. However, some pure stereoisomers may have an optical rotation that is so slight that it is undetectable with present instrumentation. The compounds of the instant invention may have one or more asymmetrical carbon atoms and therefore include various stereoisomers. All stereoisomers are included within the scope of the invention.

"Therapeutically- or pharmaceutically-effective amount" as applied to the compositions of the instant invention refers to the amount of composition sufficient to induce a desired biological result. That result can be alleviation of the signs, symptoms, or causes of a disease, or any other desired alteration of a biological system. In the present invention, the result will typically involve a decrease in the immunological and/or inflammatory responses to infection or tissue injury.

Amino acid residues in peptides are abbreviated as follows: Phenylalanine is Phe or F; Leucine is Leu or L; Isoleucine is Ile or I; Methionine is Met or M; Valine is Val or V; Serine is Ser or S; Proline is Pro or P; Threonine is Thr or T; Alanine is Ala or A; Tyrosine is Tyr or Y; Histidine is His or E; Glutamine is Gln or Q; Asparagine is Asn or N; Lysine is Lys or K; Aspartic Acid is Asp or D; Glutamic Acid is Glu or E; Cysteine is Cys or C; Tryptophan is Trp or W; Arginine is Arg or R; and Glycine is Gly or G.

In addition to peptides consisting only of naturally-occurring amino acids, peptidomimetics or peptide analogs are also provided. Peptide analogs are commonly used in the pharmaceutical industry as non-peptide drugs with properties analogous to those of the template peptide. These types of non-peptide compound are termed "peptide mimetics" or "peptidomimetics" (Fauchere, J. (1986) Adv. Drug Res. 15:29; Veber and Freidinger (1985) TINS p.392; and Evans et al. (1987) J. Med. Chem. 30:1229, which are incorporated herein by reference). Peptide mimetics that are structurally similar to therapeutically useful peptides may be used to produce an equivalent or enhanced therapeutic or prophylactic effect. Generally, peptidomimetics are structurally similar to a paradigm polypeptide (i.e., a polypeptide that has a biological or pharmacological activity), such as naturally-occurring receptor-binding polypeptide, but have one or more peptide linkages optionally replaced by a linkage selected from the group consisting of: --CH₂ NH--, --CH₂ S--, --CH₂ --CH₂ --, --CH═CH-- (cis and tras), --COCH₂ --, --CH(OH)CH₂ --, and --CH₂ SO--, by methods known in the art and further described in the following references: Spatola, A. F. in Chemistry and Biochemistry of Amino Acids, Peptides, and Proteins, B. Weinstein, eds., Marcel Dekker, New York, p. 267 (1983); Spatola, A. F., Vega Data (March 1983), Vol. 1, Issue 3, Peptide Backbone Modifications (general review); Morley, Trends Pharm Sci (1980) pp. 463-468 (general review); Hudson, D. et al., (1979) Int J Pept Prot Res 14:177-185 (--CH₂ NH--, CH₂ CH₂ --); Spatola et al., (1986) Life Sci 38:1243-1249 (--CH₂ --S); Hann (1982) J. Chem. Soc. Perkin Trans. I 307-314 (--CH--CH--, cis and trans); Almquist et al., (1980) J Med Chem 23:1392-1398 (--COCH₂ --); Jennings-White et al., (1982) Tetrahedron Lett 23:2533 (--COCH₂ --); Szelke et al., (1982) European Appln. EP 45665 CA: 97:39405 (1982) (--CH(OH)CH₂ --); Holladay et al., (1983) Tetrahedron Lett 24:4401-4404 (--C(OH)CH₂ --); and Hruby (1982) Life Sci 31:189-199 (--CH₂ --S--); each of which is incorporated herein by reference. A particularly preferred non-peptide linkage is --CH₂ NH--. Such peptide mimetics may have significant advantages over polypeptide embodiments, including, for example: more economical production, greater chemical stability, enhanced pharmacological properties (half-life, absorption, potency, efficacy, etc.), altered specificity (e.g., a broad-spectrum of biological activities), reduced antigenicity, and others. Labeling of peptidomimetics usually involves covalent attachment of one or more labels, directly or through a spacer (e.g., an amide group), to non-interfering positions) on the peptidomimetic that are predicted by quantitative structure-activity data and/or molecular modeling. Such non-interfering positions generally are positions that do not form direct contacts with the macromolecules(s) (e.g., immunoglobulin superfamily molecules) to which the peptidomimetic binds to produce the therapeutic effect. Derivitization (e.g., labeling) of peptidomimetics should not substantially interfere with the desired biological or pharmacological activity of the peptidomimetic. Generally, peptidomimetics of receptor-binding peptides bind to the receptor with high affinity and possess detectable biological activity (i.e., are agonistic or antagonistic to one or more receptor-mediated phenotypic changes).

Systematic substitution of one or more amino acids of a consensus sequence with a D-amino acid of the same type (e.g., D-lysine in place of L-lysine) may be used to generate more stable peptides. In addition, constrained peptides comprising a consensus sequence or a substantially identical consensus sequence variation may be generated by methods known in the art (Rizo and Gierasch (1992) Ann. Rev. Biochem. 61:387, incorporated herein by reference); for example, by adding internal cysteine residues capable of forming intramolecular disulfide bridges which cyclize the peptide.

Synthetic or non-naturally occuring amino acids refer to amino acids which do not naturally occur in vivo but which, nevertheless, can be incorporated into the peptide structures described herein. Preferred synthetic amino acids are the D-a-amino acids of naturally occurring L-a-amino acid as well as non-naturally occurring D- and L-a-amino acids represented by the formula H₂ NCHR⁵ COOH where R⁵ is 1) a lower alkyl group, 2) a cycloalkyl group of from 3 to 7 carbon atoms, 3) a heterocycle of from 3 to 7 carbon atoms and 1 to 2 heteroatoms selected from the group consisting of oxygen, sulfur, and nitrogen, 4) an aromatic residue of from 6 to 10 carbon atoms optionally having from 1 to 3 substituents on the aromatic nucleus selected from the group consisting of hydroxyl, lower alkoxy, amino, and carboxyl, 5) -alkylene-Y where alkylene is an alkylene group of from 1 to 7 carbon atoms and Y is selected from the group consisting of (a) hydroxy, (b) amino, (c) cycloalkyl and cycloalkenyl of from 3 to 7 carbon atoms, (d) aryl of from 6 to 10 carbon atoms optionally having from 1 to 3 substituents on the aromatic nucleus selected from the group consisting of hydroxyl, lower alkoxy, amino and carboxyl, (e) heterocyclic of from 3 to 7 carbon atoms and 1 to 2 heteroatoms selected from the group consisting of oxygen, sulfur, and nitrogen, (f) --C(O)R² where R² is selected from the group consisting of hydrogen, hydroxy, lower alkyl, lower alkoxy, and --NR³ R⁴ where R³ and R⁴ are independently selected from the group consisting of hydrogen and lower alkyl, (g) --S(O)_(n) R⁶ where n is an integer from 1 to 2 and R² is lower alkyl and with the proviso that R⁵ does not define a side chain of a naturally occurring amino acid.

Other preferred synthetic amino acids include amino acids wherein the amino group is separated from the carboxyl group by more than one carbon atom such as b-alanine, g-aminobutyric acid, and the like.

Particularly preferred synthetic amino acids include, by way of example, the D-amino acids of naturally occurring L-amino acids, L-1-napthyl-alanine, L-2-naphthylalanine, L-cyclohexylalanine, L-2-amino isobutyric acid, the sulfoxide and sulfone derivatives of methionine (i.e., HOOC--(H₂ NCH)CH₂ CH₂ --S(O)_(n) R⁶) where n and R₆ are as defined above as well as the lower alkoxy derivative of methionine (i.e., HOOC--(H₂ NCH)CH₂ CH₂ --OR⁶ where R⁶ is as defined above).

"Detectable label" refers to materials, which when covalently attached to the peptides and peptide mimetics of this invention, permit detection of the peptide and peptide mimetics in vivo in the patient to whom the peptide or peptide mimetic has been administered. Suitable detectable labels are well known in the art and include, by way of example, radioisotopes, fluorescent labels (e.g., fluorescein), and the like. The particular detectable label employed is not critical and is selected relative to the amount of label to be employed as well as the toxicity of the label at the amount of label employed. Selection of the label relative to such factors is well within the skill of the art.

Covalent attachment of the detectable label to the peptide or peptide mimetic is accomplished by conventional methods well known in the art. For example, when the ¹²⁵ I radioisotope is employed as the detectable label, covalent attachment of ¹²⁵ I to the peptide or the peptide mimetic can be achieved by incorporating the amino acid tyrosine into the peptide or peptide mimetic and then iodating the peptide. If tyrosine is not present in the peptide or peptide mimetic, incorporation of tyrosine to the N or C terminus of the peptide or peptide mimetic can be achieved by well known chemistry. Likewise, ³² P can be incorporated onto the peptide or peptide mimetic as a phosphate moiety through, for example, a hydroxyl group on the peptide or peptide mimetic using conventional chemistry.

II. Overview

The present invention provides compounds that bind to and activate the TPO-R or otherwise behave as a TPO agonist. These compounds include "lead" peptide compounds and "derivative" compounds constructed so as to have the same or similar molecular structure or shape as the lead compounds but that differ from the lead compounds either with respect to susceptibility to hydrolysis or proteolysis and/or with respect to other biological properties, such as increased affinity for the receptor. The present invention also provides compositions comprising an effective amount of a TPO agonist, and more particularly a compound, that is useful for treating hematological disorders, and particularly, thrombocytopenia associated with chemotherapy, radiation therapy, or bone marrow transfusions.

III. Identification of TPO-Agonists

Peptides having a binding affinity to TPO-R can be readily identified by random peptide diversity generating systems coupled with an affinity enrichment process.

Specifically, random peptide diversity generating systems include the "peptides on plasmids" system described in U.S. Pat. Nos. 5,270,170 and 5,338,665; the "peptides on phage" system described in U.S. patent application Ser. No. 07/718,577, filed Jun. 20, 1991 which is a continuation in part application of U.S. patent application Ser. No. 07/541,108, filed Jun. 20, 1990, and in Cwirla et al., (1990) Proc. Natl. Acad. Sci. USA 87:6378-6382; the "encoded synthetic library (ESL)" system described in U.S. patent application Ser. No. 08/146,886, filed Nov. 12, 1993 which is a continuation in part application of U.S. Ser. No. 07/946,239, filed Sep. 16, 1992, which is a continuation in part application of U.S. Ser. No. 07/762,522, filed Sep. 18, 1991; and the "very large scale immobilized polymer synthesis" system described in U.S. Pat. No. 5,143,854; PCT patent publication Ser. No. 90/15070, published Dec. 13, 1990; U.S. patent application Ser. No. 624,120, filed Dec. 6, 1990; Fodor et al., Feb. 15, 1991, Science 251:767-773; Dower and Fodor (1991) Ann. Rep. Med. Chem. 26:271-180; and U.S. patent application Ser. No. 805,727, filed Dec. 6, 1991.

Using the procedures described above, random peptides were generally designed to have a defined number of amino acid residues in length (e.g., 12). To generate the collection of oligonucleotides encoding the random peptides, the codon motif (NNK)x, where N is nucleotide A, C, G, or T (equimolar; depending on the methodology employed, other nucleotides can be employed), K is G or T (equimolar), and x is an integer corresponding to the number of amino acids in the peptide (e.g., 12) was used to specify any one of the 32 possible codons resulting from the NNX motif: 1 for each of 12 amino acids, 2 for each of 5 amino acids, 3 for each of 3 amino acids, and only one of the three stop codons. Thus, the NNK motif encodes all of the amino acids, encodes only one stop codon, and reduces codon bias.

In the systems employed, the random peptides were presented either on the surface of a phage particle, as part of a fusion protein comprising either the pIII or the pVIII coat protein of a phage fd derivative (peptides on phage) or as a fusion protein with the LacI peptide fusion protein bound to a plasmid (peptides on plasmids). The phage or plasmids, including the DNA encoding the peptides, were identified and isolated by an affinity enrichment process using immobilized TPO-R. The affinity enrichment process, sometimes called "panning," involves multiple rounds of incubating the phage or plasmids with the immobilized receptor, collecting the phage or plasmids that bind to the receptor (along with the accompanying DNA), and producing more of the phage or plasmids (along with the accompanying LacI-peptide fusion protein) collected. The extracellular domain (ECD) of the TPO-R typically was used during panning.

After several rounds of affinity enrichment, the phage or plasmids and accompanying peptides were examined by ELISA to determine if the peptides bind specifically to TPO-R. This assay was carried out similarly to the procedures used in the affinity enrichment process, except that after removing unbound phage, the wells were typically treated with rabbit anti-phage antibody, then with alkaline phosphatase (AP)-conjugated goat anti-rabbit antibody. The amount of alkaline phosphatase in each well was determined by standard methods. A similar ELISA procedure for use in the peptides on plasmids system is described in detail below.

By comparing test wells with control wells (no receptor), one can determine whether the fusion proteins bind to the receptor specifically. The phage pools found to bind to TPO-R were screened in a colony lift probing format using radiolabeled monovalent receptor. This probe can be produced using protein kinase A to phosphorylate a kemptide sequence fused to the C-terminus of the soluble receptor. The "engineered" form of the TPO receptor is then expressed in host cells, typically CHO cells. Following PI-PLC harvest of the receptors, the receptor was tested for binding to TPO or TPO-R specific phage clones. The receptor is then labeled to high specific activity with ³³ P for use as a monovalent probe to identify high affinity ligands using colony lifts.

Peptides found to bind specifically to the receptor were then synthesized as the free peptide (e.g., no phage) and tested in a blocking assay. The blocking assay was carried out in similar fashion to the ELISA, except that TPO or a reference peptide was added to the wells before the fusion protein (the control wells were of two types: (1) no receptor; and (2) no TPO or reference peptide). Fusion proteins for which the binding to the receptor was blocked by TPO or the reference peptide contain peptides in the random peptide portion that are preferred compounds of the invention.

TPO-R, as well as its extracellular domain, were produced in recombinant host cells. One useful form of TPO-R is constructed by expressing the protein as a soluble protein in baculovirus transformed host cells using standard methods; another useful form is constructed with a signal peptide for protein secretion and for glycophospholipid membrane anchor attachment. This form of anchor attachment is called "PIG-tailing". See Caras and Wendell (1989) Science 243:1196-1198 and Lin et al. (1990) Science 249:677-679.

Using the PIG-tailing system, one can cleave the receptor from the surface of the cells expressing the receptor (e.g., transformed CEO cells selected for high level expression of receptor with a cell sorter) with phospholipase C. The cleaved receptor still comprises a carboxy terminal sequence of amino acids, called the "HPAP tall", from the signal protein for membrane attachment and can be Immobilized without further purification. The recombinant receptor protein can be immobilized by coating the wells of microtiter plates with an anti-HPAP tail antibody (Ab 179 or MAb 179), blocking non-specific binding with bovine serum albumin (BSA) in PES, and then binding cleaved recombinant receptor to the antibody. Using this procedure, one should perform the immobilization reaction in varying concentrations of receptor to determine the optimum amount for a given preparation, because different preparations of recombinant protein often contain different amounts of the desired protein. In addition, one should ensure that the immobilizing antibody is completely blocked (with TPO or some other blocking compound) during the affinity enrichment process. Otherwise, unblocked antibody can bind undesired phage during the affinity enrichment procedure. One can use peptides that bind to the immobilizing antibody to block unbound sites that remain after receptor immobilization to avoid this problem or one can simply immobilize the receptor directly to the wells of microtiter plates, without the aid of an immobilizing antibody. See U.S. patent application Ser. No. 07/947,339, filed Sep. 18, 1992, incorporated herein by reference.

When using random peptide generation systems that allow for multivalent ligand-receptor interaction, one must recognize that the density of the immobilized receptor is an important factor in determining the affinity of the ligands that can bind to the immobilized receptor. At higher receptor densities (e.g., each anti-receptor antibody-coated well treated with 0.25 to 0.5 mg of receptor), multivalent binding is more likely to occur than at lower receptor densities (e.g., each anti-receptor antibody-coated well treated with 0.5 to 1 ng of the receptor). If multivalent binding is occurring, then one will be more likely to isolate ligands with relatively lower affinity, unless one uses high densities of immobilized receptor to identify lead compounds and uses lower receptor densities to isolate higher affinity derivative compounds.

Preferred screening methods to facilitate identification of peptides which bind TPO-R involve first identifying lead peptides which bind to the extracellular domain of the receptor and then making other peptides which resemble the lead peptides. Specifically, using a pIII or pVIII-based peptides on phage system, a random library can be screened to discover a phage that presents a peptide that binds to TPO-R. The phage DNAs are sequenced to determine the sequences of the peptides displayed on the surface of the phages.

Clones capable of specific binding to the TPO-R were identified from a random linear 10-mer pVIII library and a random linear 11-mer pVIII library. The sequences of these peptides serve as the basis for the construction of other peptide libraries designed to contain a high frequency of derivatives of the initially identified peptides. These libraries can be synthesized so as to favor the production of peptides that differ from the binding peptide in only a few residues. This approach involves the synthesis of an oligonucleotide with the binding peptide coding sequence, except that rather than using pure preparations of each of the four nucleoside triphosphates in the synthesis, one uses mixtures of the four nucleoside triphosphates (i.e., 55% of the "correct" nucleotide, and 15% each of the other three nucleotides is one preferred mixture for this purpose and 70% of the "correct" nucleotide and 10% of each of the other three nucleotides is another preferred mixture for this purpose) so as to generate derivatives of the binding peptide coding sequence.

A variety of strategies were used to derivatize the lead peptides by making "mutagenesis on a theme" libraries, including a pVIII phagemid mutagenesis library based on the consensus sequence and mutagenized at 70:10:10:10 frequency with 5 NNK codons on each terminus (probing with radiolabeled monovalent receptor and with or without peptide elution).

The "peptides on plasmids" techniques was also used for peptide screening and mutagenesis studies and is described in greater detail in U.S. Pat. No. 5,338,665, which is incorporated herein by reference for all purposes. According to this approach, random peptides are fused at the C-terminus of LacI through expression from a plasmid vector carrying the fusion gene. Linkage of the LacI-peptide fusion to its encoding DNA occurs via the lacO sequences on the plasmid, forming a stable peptide-LacI-plasmid complex that can be screened by affinity purification (panning) on an immobilized receptor. The plasmids thus isolated can then be reintroduced into E. coli by electroporation to amplify the selected population for additional rounds of screening, or for the examination of individual clones.

In addition, random peptide screening and mutagenesis studies were performed using a modified C-terminal Lac-I display system in which display valency was reduced ("headpiece dimer" display system). The libraries were screened and the resulting DNA inserts were cloned as a pool into a maltose binding protein (MBP) vector allowing their expression as a C-terminal fusion protein. Crude cell lysates from randomly picked individual MBP fusion clones were then assayed for TPO-R binding in an ELISA format, as discussed above.

Screening of the various libraries described above yielded the TPO receptor binding peptides shown in Table 1 below, as well as others not listed herein.

                                      TABLE 1     __________________________________________________________________________     Peptide     __________________________________________________________________________     G R V R D Q V A G W (SEQ ID. NO. 6)     G R V K D Q I A Q L (SEQ ID. NO. 7)     G V R D Q V S W A L (SEQ ID. NO. 8)     E S V R E Q V M K Y (SEQ ID. NO. 9)     S V R S Q I S A S L (SEQ ID. NO. 10)     G V R D Q V S W A L (SEQ ID. NO. 11)     G V R E T V Y R H M (SEQ ID. NO. 12)     G V R E V I V M H M L (SEQ ID. NO. 13)     S D L R D C I L M L H (SEQ ID. NO. 14)     A G V R D Q I L I W L (SEQ ID. NO. 15)     G R V R D Q I M L S L (SEQ ID. NO. 16)     G V R Q Q V F A Y L V (SEQ ID. NO. 17)     L S V R D Q V M A W L (SEQ ID. NO. 18)     G K V R D Q V Y L H L (SEQ ID. NO. 19)     L S V R E Q V M A W L (SEQ ID. NO. 20)     G R V R D Q I W A A L (SEQ ID. NO. 21)     S S V R E Q V C L F F (SEQ ID. NO. 22)     G R V R E Q V A Y C L (SEQ ID. NO. 23)     T G L R S V R D Q V C A H F A (SEQ ID. NO. 24)     Y S S G S V R E Q V M A F L A (SEQ ID. NO. 25)     V G S N S V R D Q V T A W L L (SEQ ID. NO. 26)     A T G N R V R E Q V F W Y F L (SEQ ID. NO. 27)     G Y R Y S V G E Q V A M W L A (SEQ ID. NO. 28)     Y V R E S V R D Q V W K F Y C (SEQ ID. NO. 29)     S G S L S V R E Q V H L W L A (SEQ ID. NO. 30)     S E R Q S V R E Q V S Q W L L (SEQ ID. NO. 31)     Q V R G S V R E Q V M I W L S (SEQ ID. NO. 32)     G S N Q S V R E Q V W A Y V M (SEQ ID. NO. 33)     R Q L Y S V R D Q V A M Y L M (SEQ ID. NO. 34)     R P W T S V R E Q V Y L H L I (SEQ ID. NO. 35)     Y S R G S V R D Q V F S W L H (SEQ ID. NO. 36)     D M V Q S V R E Q V A L F L A (SEQ ID. NO. 37)     E T R T S V R E Q V A A F L A (SEQ ID. NO. 38)     R S S V S V R D Q V L A F L S (SEQ ID. NO. 39)     N P G L S V R D Q V F L H F F (SEQ ID. NO. 40)     W T R I S V R D Q V W A F Y H (SEQ ID. NO. 41)     R P T S R V R E Q V M L W L S (SEQ ID. NO. 42)     S R G A S V R D Q V V L H L C (SEQ ID. NO. 43)     A G V P S V R E Q V V A R L C (SEQ ID. NO. 44)     G E T R S V R D Q V I A R L C (SEQ ID. NO. 45)     S G R Q S V R D Q V L G C T W (SEQ ID. NO. 46)     S H M S V R E Q V Q Y Y L L    (SEQ ID. NO. 47)     S A C C R V R D Q V F W F W H (SEQ ID. NO. 48)     S Q L A T V R E Q V Y A F W C (SEQ ID. NO. 49)     A K N L S V R E Q V M L H L C (SEQ ID. NO. 50)     S G P D S V R E Q V I S R L C (SEQ ID. NO. 51)     T E R L S V R E Q V L C W V T (SEQ ID. NO. 52)     W T R T S V R D Q V A L W L H (SEQ ID. NO. 53)     H R G P S V R D Q V C S W W L (SEQ ID. NO. 54)     E G X N Y V R E Q V C A F F Y (SEQ ID. NO. 55)     A T T R S V R E Q V Y L F L S (SEQ ID. NO. 56)     R V G L S V R D Q V Y A W L A (SEQ ID. NO. 57)     __________________________________________________________________________

IC₅₀ values for some additional representative peptides are given in the table below. A variety of methods can be used to evaluate IC₅₀ values. For example, an equilibrium binding ELISA assay, using either MBP-TPO or lacI-peptide tracer, was used to determine whether the peptides inhibit the binding of TPO to the extracellular domain of the TPO receptor. The IC₅₀ value can be determined using the free peptide, which optionally can be C-terminally amidated, or can be prepared as an ester or other carboxy amide. To recreate the exact sequence displayed by the phage, the N-terminal and C-terminal amino acids of the synthetic peptides are often preceded by one or two glycine residues. These glycines are not believed to be necessary for binding or activity.

IC₅₀ values are indicated symbolically by the symbols "-", "+", and "++". For examples, those peptides which showed IC₅₀ values in excess of 100 μM are indicated with a "-". Those peptides which gave IC₅₀ values of less than or equal to 100 μM are given a "+", while those which gave IC₅₀ values of 500 nm or less are indicated with a "++". Those peptides which gave IC₅₀ values at or near the cutoff point for a particular symbol are indicated with a hybrid designator, e.g., "+/-". Those peptides for which IC₅₀ values were not determined are listed as "N.D.".

                                      TABLE 2     __________________________________________________________________________     Peptide                            Affinity     __________________________________________________________________________     G R V R D Q I M L S L G G (SEQ. ID. NO. 58)                                        -     Q D P R G V R E Q V I A R L C      ++                                 |     Q D P R G V R E Q V I A R L C     __________________________________________________________________________

The tables above illustrate that a preferred core peptide comprises a sequence of amino acids:

    V R X.sub.1 Q X.sub.2 X X X X.sub.3 (SEQ. ID. NO.1)

where X₁ is D or E; X₂ is I or V; X₃ is F, E, L, M, T, V, W, or Y; and each X residue is independently selected from any of the 20 genetically coded L-amino acids, their stereoisomeric D-amino acids; and non-natural amino acids. Preferably, each X residue is independently selected from any of the 20 genetically coded L-amino acids and their stereoisomeric D-amino acids. More preferably, X₃ is F, L, W, or Y.

In a preferred embodiment, the core peptide comprises a sequence of amino acids:

    V R X.sub.1 Q X.sub.2 X.sub.4 X.sub.5 X.sub.6 X.sub.3 (SEQ. ID. NO.2)

where X₄ is A, C, F, H, I, L, M, Q, S, T, V, W, or Y; X₅ is A, C, G, I, K, L, M, Q, R, S, W, or V; and X₆ is A, C, F, H, L, R, S, W, or Y. More preferably, X₄ is A, C, F, I, L, M, S, W, or Y; X₅ is A, L, M, or W; and X₆ is F, H, R, W, or Y.

In a more preferred embodiment, the core peptide comprises a sequence of amino acids:

    X X V R X.sub.1 Q X.sub.2 X.sub.4 X.sub.5 X.sub.6 X.sub.3 (SEQ. ID. NO.3)

where each X residue is independently selected from any of the 20 genetically coded L-amino acids, their stereoisomeric D-amino acids; and non-natural amino acids. Preferably, each X residue is independently selected from any of the 20 genetically coded L-amino acids and their stereoisomeric D-amino acids.

More preferably, the core peptide comprises a sequence of amino acids:

    X.sub.7 X.sub.8 V R X.sub.1 Q X.sub.2 X.sub.4 X.sub.5 X.sub.6 X.sub.3 (SEQ. ID. NO.4)

where X₇ is A, E, G, I, L, N, Q, P, R, S, T, V, or Y; and X₈ is D, E, G, K, R, S, T, or Y. Even more preferably, X₇ is E, G L, R, S, or T; and X₈ is E, G, R, or S.

Particularly preferred peptides include: Q D P R G V R E Q V I A R L C (SEQ. ID. NO.5) and its disulfide linked homodimer:

    QDPRGVREQVIARLC                   |     QDPRGVREQVIARLC

Peptides and peptidomimetics having an IC₅₀ of greater than about 100 mM lack sufficient binding to permit use in either the diagnostic or therapeutic aspects of this invention. Preferably, for diagnostic purposes, the peptides and peptidomimetics have an IC₅₀ of about 2 mM or less and, for pharmaceutical purposes, the peptides and peptidomimetics have an IC₅₀ of about 100 mM or less.

The binding peptide sequence also provides a means to determine the minimum size of a TPO-R binding compound of the invention. Using the "encoded synthetic library" (ESL) system or the "very large scale immobilized polymer synthesis" system, one can not only determine the minimum size of a peptide with such activity, but one can also make all of the peptides that form the group of peptides that differ from the preferred motif (or the minimum size of that motif) in one, two, or more residues. This collection of peptides can then be screened for ability to bind to TPO-receptor. These immobilized polymers synthesis systems or other peptide synthesis methods can also be used to synthesize truncation analogs, deletion analogs, substitution analogs, and combinations thereof all of the peptide compounds of the invention.

The peptides and peptide mimetics of the present invention were also evaluated in a thrombopoietin dependent cell proliferation assay, as described in greater detail in Example 2 below. Cell proliferation is measured by techniques known in the art, such as an MTT assay which correlates with ³ -thymidine incorporation as an indication of cell proliferation (see, Mossmann (1983) J. Immunol. Methods 65:55). The peptides tested stimulated proliferation of TPO-R transfected Ba/F3 cells in a dose dependent manner as shown in FIG. 1. These peptides have no effect on the parental cell line.

IV. Preparation of Peptides and Peptide Mimetics

A. Solid Phase Synthesis

The peptides of the invention can be prepared by classical methods known in the art, for example, by using standard solid phase techniques. The standard methods include exclusive solid phase synthesis, partial solid phase synthesis methods, fragment condensation, classical solution synthesis, and even by recombinant DNA technology. See, e.g., Merrifield (1963) J. Am. Chem. Soc. 85:2149, incorporated herein by reference. On solid phase, the synthesis is typically commenced from the C-terminal end of the peptide using an alpha-amino protected resin. A suitable starting material can be prepared, for instance, by attaching the required alpha-amino acid to a chloromethylated resin, a hydroxymethyl resin, or a benzhydrylamine resin. One such chloromethylated resin is sold under the tradename BIO-BEADS SX-1 by Bio Rad Laboratories, Richmond, Calif., and the preparation of the hydroxymethyl resin is described by Bodonszky et al., (1966) Chem. Ind. (London) 38:1597. The benzhydrylamine (BHA) resin has been described by Pietta and Marshall (1970) Chem. Commn. 650, and is commercially available from Beckman Instruments, Inc., Palo Alto, Calif., in the hydrochloride form.

Thus, the compounds of the invention can be prepared by coupling an alpha-amino protected amino acid to the chloromethylated resin with the aid of, for example, cesium bicarbonate catalyst, according to the method described by Gisin (1973) Helv. Chim. Acta 56:1467. After the initial coupling, the alpha-amino protecting group is removed by a choice of reagents including trifluoroacetic acid (TFA) or hydrochloric acid (HCl) solutions in organic solvents at room temperature.

The alpha-amino protecting groups are those known to be useful in the art of stepwise synthesis of peptides. Included are acyl type protecting groups (e.g., formyl, trifluoroacetyl, acetyl), aromatic urethane type protecting groups (e.g., benzyloxycarboyl (Cbz) and substituted Cbz), aliphatic urethane protecting groups (e.g., t-butyloxycarbonyl (Boc), isopropyloxycarbonyl, cyclohexyloxycarbonyl) and alkyl type protecting groups (e.g., benzyl, triphenylmethyl). Boc and Fmoc are preferred protecting groups. The side chain protecting group remains intact during coupling and is not split off during the deprotection of the amino-terminus protecting group or during coupling. The side chain protecting group must be removable upon the completion of the synthesis of the final peptide and under reaction conditions that will not alter the target peptide.

The side chain protecting groups for Tyr include tetrahydropyranyl, tert-butyl, trityl, benzyl, Cbz, Z--Br--Cbz, and 2,5-dichlorobenzyl. The side chain protecting groups for Asp include benzyl, 2,6-dichlorobenzyl, methyl, ethyl, and cyclohexyl. The side chain protecting groups for Thr and Ser include acetyl, benzoyl, trityl, tetrahydropyranyl, benzyl, 2,6-dichlorobenzyl, and Cbz. The side chain protecting group for Thr and Ser is benzyl. The side chain protecting groups for Arg include nitro, Tosyl (Tos), Cbz, adamantyloxycarbonyl mesitoylsulfonyl (Mts), or Boc. The side chain protecting groups for Lys include Cbz, 2-chlorobenzyloxycarbonyl (2-Cl--Cbz), 2-bromobenzyloxycarbonyl (2-BrCbz), Tos, or Boc.

After removal of the alpha-amino protecting group, the remaining protected amino acids are coupled stepwise in the desired order. An excess of each protected amino acid is generally used with an appropriate carboxyl group activator such as dicyclohexylcarbodiimide (DCC) in solution, for example, in methylene chloride (CH₂ Cl₂), dimethyl formamide (DMF) mixtures.

After the desired amino acid sequence has been completed, the desired peptide is decoupled from the resin support by treatment with a reagent such as trifluoroacetic acid or hydrogen fluoride (HF), which not only cleaves the peptide from the resin, but also cleaves all remaining side chain protecting groups. When the chloromethylated resin is used, hydrogen fluoride treatment results in the formation of the free peptide acids. When the benzhydrylamine resin is used, hydrogen fluoride treatment results directly in the free peptide amide. Alternatively, when the chloromethylated resin is employed, the side chain protected peptide can be decoupled by treatment of the peptide resin with ammonia to give the desired side chain protected amide or with an alkylamine to give a side chain protected alkylamide or dialkylamide. Side chain protection is then removed in the usual fashion by treatment with hydrogen fluoride to give the free amides, alkylamides, or dialkylamides.

These solid phase peptide synthesis procedures are well known in the art and further described in Stewart, Solid Phase Peptide Syntheses (Freeman and Co., San Francisco, 1969).

Using the "encoded synthetic library" or "very large scale immobilized polymer synthesis" system described in U.S. patent application Ser. Nos. 492,462, filed Mar. 7, 1990; 624,120, filed Dec. 6, 1990; and 805,727, filed Dec. 6, 1991; one can not only determine the minimum size of a peptide with such activity, one can also make all of the peptides that form the group of peptides that differ from the preferred motif (or the minimum size of that motif) in one, two, or more residues. This collection of peptides can then be screened for ability to bind to TPO-R. This immobilized polymer synthesis system or other peptide synthesis methods can also be used to synthesize truncation analogs and deletion analogs and combination of truncation and deletion analogs of all of the peptide compounds of the invention.

B. Synthetic Amino Acids

These procedures can also be used to synthesize peptides in which amino acids other than the 20 naturally occurring, genetically encoded amino acids are substituted at one, two, or more positions of any of the compounds of the invention. For instance, naphthylalanine can be substituted for tryptophan, facilitating synthesis. Other synthetic amino acids that can be substituted into the peptides of the present invention include L-hydroxypropyl, L-3, 4-dihydroxyphenylalanyl, d amino acids such as L-d-hydroxylysyl and D-d-methylalanyl, L-a-methylalanyl, b amino acids, and isoquinolyl. D amino acids and non-naturally occurring synthetic amino acids can also be incorporated into the peptides of the present invention.

One can replace the naturally occurring side chains of the 20 genetically encoded amino acids (or D amino acids) with other side chains, for instance with groups such as alkyl, lower alkyl, cyclic 4-, 5-, 6-, to 7-membered alkyl, amide, amide lower alkyl, amide di(lower alkyl), lower alkoxy, hydroxy, carboxy and the lower ester derivatives thereof, and with 4-, 5-, 6-, to 7-membered hetereocyclic. In particular, proline analogs in which the ring size of the proline residue is changed from 5 members to 4, 6, or 7 members can be employed. Cyclic groups can be saturated or unsaturated, and if unsaturated, can be aromatic or non-aromatic.

Cyclic groups can be saturated or unsaturated, and if unsaturated, can be aromatic or non-aromatic. Heterocyclic groups preferably contain one or more nitrogen, oxygen, and/or sulphur heteroatoms. Examples of such groups include the furazanyl, furyl, imidazolidinyl, imidazolyl, imidazolinyl, isothiazolyl, isoxazolyl, morpholinyl (e.g., morpholino), oxazolyl, piperazinyl (e.g., 1-piperazinyl), piperidyl (e.g., 1-piperidyl, piperidino), pyranyl, pyrazinyl, pyrazolidinyl, pyrazolinyl, pyrazolyl, pyridazinyl, pyridyl, pyrimidinyl, pyrrolidinyl (e.g., 1-pyrrolidinyl), pyrrolinyl, pyrrolyl, thiadiazolyl, thiazolyl, thienyl, thiomorpholinyl (e.g., thiomorpholino), and triazolyl. These heterocyclic groups can be substituted or unsubstituted. Where a group is substituted, the substituent can be alkyl, alkoxy, halogen, oxygen, or substituted or unsubstituted phenyl.

One can also readily modify the peptides of the instant invention by phosphorylation, and other methods for making peptide derivatives of the compounds of the present invention are described in Hruby et al.⁴² Thus, the peptide compounds of the invention also serve as a basis to prepare peptide mimetics with similar biological activity.

C. Terminal Modifications

Those of skill in the art recognize that a variety of techniques are available for constructing peptide mimetics with the same or similar desired biological activity as the corresponding peptide compound but with more favorable activity than the peptide with respect to solubility, stability, and susceptibility to hydrolysis and proteolysis. See, for example, Morgan and Gainor (1989) Ann. Rep. Med. Chem. 24:243-252. The following describes methods for preparing peptide mimetics modified at the N-terminal amino group, the C-terminal carboxyl group, and/or changing one or more of the amido linkages in the peptide to a non-amido linkage. It being understood that two or more such modifications can be coupled in one peptide mimetic structure (e.g., modification at the C-terminal carboxyl group and inclusion of a --CH₂ -carbamate linkage between two amino acids in the peptide).

1. N-Terminal Modifications

The peptides typically are synthesized as the free acid but, as noted above, could be readily prepared as the amide or ester. One can also modify the amino and/or carboxy terminus of the peptide compounds of the invention to produce other compounds of the invention. Amino terminus modifications include methylating (i.e., --NHCH₃ or --NH(CH₃)₂), acetylating, adding a carbobenzoyl group, or blocking the amino terminus with any blocking group containing a carboxylate functionality defined by RCOO--, where R is selected from the group consisting of naphthyl, acridinyl, steroidyl, and similar groups. Carboxy terminus modifications include replacing the free acid with a carboxamide group or forming a cyclic lactam at the carboxy terminus to introduce structural constraints.

Amino terminus modifications are as recited above and include alkylating, acetylating, adding a carbobenzoyl group, forming a succinimide group, etc. Specifically, the N-terminal amino group can then be reacted as follows:

(a) to form an amide group of the formula RC(O)NH-- where R is as defined above by reaction with an acid halide e.g., RC(O)Cl! or acid anhydride. Typically, the reaction can be conducted by contacting about equimolar or excess amounts (e.g., about 5 equivalents) of an acid halide to the peptide in an inert diluent (e.g., dichloromethane) preferably containing an excess (e.g., about 10 equivalents) of a tertiary amine, such as diisopropylethylamine, to scavenge the acid generated during reaction. Reaction conditions are otherwise conventional (e.g., room temperature for 30 minutes). Alkylation of the terminal amino to provide for a lower alkyl N-substitution followed by reaction with an acid halide as described above will provide for N-alkyl amide group of the formula RC(O)NR--;

(b) to form a succinimide group by reaction with succinic anhydride. As before, an approximately equimolar amount or an excess of succinic anhydride (e.g., about 5 equivalents) can be employed and the amino group is converted to the succinimide by methods well known in the art including the use of an excess (e.g., ten equivalents) of a tertiary amine such as diisopropylethylamine in a suitable inert solvent (e.g., dichloromethane). See, for example, Wollenberg, et al., U.S. Pat. No. 4,612,132 which is incorporated herein by reference in its entirety. It is understood that the succinic group can be substituted with, for example, C₂ -C₆ alkyl or --SR substituents which are prepared in a conventional manner to provide for substituted succinimide at the N-terminus of the peptide. Such alkyl substituents are prepared by reaction of a lower olefin (C₂ -C₆) with maleic anhydride in the manner described by Wollenberg, et al., supra. and --SR substituents are prepared by reaction of RSH with maleic anhydride where R is as defined above;

(c) to form a benzyloxycarbonyl-NH-- or a substituted benzyloxycarbonyl-NH-- group by reaction with approximately an equivalent amount or an excess of CBZ--Cl (i.e., benzyloxycarbonyl chloride) or a substituted CBZ--Cl in a suitable inert diluent (e.g., dichloromethane) preferably containing a tertiary amine to scavenge the acid generated during the reaction;

(d) to form a sulfonamide group by reaction with an equivalent amount or an excess (e.g., 5 equivalents) of R--S(O)₂ Cl in a suitable inert diluent (dichloromethane) to convert the terminal amine into a sulfonamide where R is as defined above. Preferably, the inert diluent contains excess tertiary amine (e.g., ten equivalents) such as diisopropylethylamine, to scavenge the acid generated during reaction. Reaction conditions are otherwise conventional (e.g., room temperature for 30 minutes);

(e) to form a carbamate group by reaction with an equivalent amount or an excess (e.g., 5 equivalents) of R--OC(O)Cl or R--OC(O)OC₆ H₄ --p--NO₂ in a suitable inert diluent (e.g., dichloromethane) to convert the terminal amine into a carbamate where R is as defined above. Preferably, the inert diluent contains an excess (e.g., about 10 equivalents) of a tertiary amine, such as diisopropylethylamine, to scavenge any acid generated during reaction. Reaction conditions are otherwise conventional (e.g., room temperature for 30 minutes); and

(f) to form a urea group by reaction with an equivalent amount or an excess (e.g., 5 equivalents) of R--N═C═O in a suitable inert diluent (e.g., dichloromethane) to convert the terminal amine into a urea (i.e., RNHC(O)NH--) group where R is as defined above. Preferably, the inert diluent contains an excess (e.g., about 10 equivalents) of a tertiary amine, such as diisopropylethylamine. Reaction conditions are otherwise conventional (e.g., room temperature for about 30 minutes).

2. C-Terminal Modifications

In preparing peptide mimetics wherein the C-terminal carboxyl group is replaced by an ester (i.e., --C(O)OR where R is as defined above), the resins used to prepare the peptide acids are employed, and the side chain protected peptide is cleaved with base and the appropriate alcohol, e.g., methanol. Side chain protecting groups are then removed in the usual fashion by treatment with hydrogen fluoride to obtain the desired ester.

In preparing peptide mimetics wherein the C-terminal carboxyl group is replaced by the amide --C(O)NR³ R⁴, a benzhydrylamine resin is used as the solid support for peptide synthesis. Upon completion of the synthesis, hydrogen fluoride treatment to release the peptide from the support results directly in the free peptide amide (i.e., the C-terminus is --C(O)NH₂). Alternatively, use of the chloromethylated resin during peptide synthesis coupled with reaction with ammonia to cleave the side chain protected peptide from the support yields the free peptide amide and reaction with an alkylamine or a dialkylamine yields a side chain protected alkylamide or dialkylamide (i.e., the C-terminus is --C(O)NRR¹ where R and R¹ are as defined above). Side chain protection is then removed in the usual fashion by treatment with hydrogen fluoride to give the free amides, alkylamides, or dialkylamides.

In another alternative embodiment, the C-terminal carboxyl group or a C-terminal ester can be induced to cyclize by internal displacement of the --OH or the ester (--OR) of the carboxyl group or ester respectively with the N-terminal amino group to form a cyclic peptide. For example, after synthesis and cleavage to give the peptide acid, the free acid is converted to an activated ester by an appropriate carboxyl group activator such as dicyclohexylcarbodiimide (DCC) in solution, for example, in methylene chloride (CH₂ Cl₂), dimethyl formamide (DMF) mixtures. The cyclic peptide is then formed by internal displacement of the activated ester with the N-terminal amine. Internal cyclization as opposed to polymerization can be enhanced by use of very dilute solutions. Such methods are well known in the art.

One can also cyclize the peptides of the invention, or incorporate a desamino or descarboxy residue at the terminii of the peptide, so that there is no terminal amino or carboxyl group, to decrease susceptibility to proteases or to restrict the conformation of the peptide. C-terminal functional groups of the compounds of the present invention include amide, amide lower alkyl, amide di(lower alkyl), lower alkoxy, hydroxy, and carboxy, and the lower ester derivatives thereof, and the pharmaceutically acceptable salts thereof.

D. Backbone Modifications

Other methods for making peptide derivatives of the compounds of the present invention are described in Hruby et al., (1990) Biochem J. 268(2):249-262, incorporated herein by reference. Thus, the peptide compounds of the invention also serve as structural models for non-peptidic compounds with similar biological activity. Those of skill in the art recognize that a variety of techniques are available for constructing compounds with the same or similar desired biological activity as the lead peptide compound but with more favorable activity than the lead with respect to solubility, stability, and susceptibility to hydrolysis and proteolysis. See Morgan and Gainor (1989) Ann. Rep. Med. Chem. 24:243-252, incorporated herein by reference. These techniques include replacing the peptide backbone with a backbone composed of phosphonates, amidates, carbamates, sulfonamides, secondary amines, and N-methylamino acids.

Peptide mimetics wherein one or more of the peptidyl linkages --C(O)NH--! have been replaced by such linkages as a --CH₂ -carbamate linkage, a phosphonate linkage, a --CH₂ -sulfonamide linkage, a urea linkage, a secondary amine (--CH₂ NH--) linkage, and an alkylated peptidyl linkage --C(O)NR⁶ -- where R⁶ is lower alkyl! are prepared during conventional peptide synthesis by merely substituting a suitably protected amino acid analogue for the amino acid reagent at the appropriate point during synthesis.

Suitable reagents include, for example, amino acid analogues wherein the carboxyl group of the amino acid has been replaced with a moiety suitable for forming one of the above linkages. For example, if one desires to replace a --C(O)NR-- linkage in the peptide with a --CH₂ -carbamate linkage (--CH₂ OC(O)NR--), then the carboxyl (--COOH) group of a suitably protected amino acid is first reduced to the --CH₂ OH group which is then converted by conventional methods to a --OC(O)Cl functionality or a para-nitrocarbonate --OC(O)O--C₆ H₄ -p--NO₂ functionality. Reaction of either of such functional groups with the free amine or an alkylated amine on the N-terminus of the partially fabricated peptide found on the solid support leads to the formation of a --CH₂ OC(O)NR-- linkage. For a more detailed description of the formation of such --CH₂ -carbamate linkages, see Cho et al, Science, 261:1303-1305 (1993).

Similarly, replacement of an amido linkage in the peptide with a phosphonate linkage can be achieved in the manner set forth in U.S. patent application Ser. Nos. 07/943,805, 08/081,577, and 08/119,700, the disclosures of which are incorporated herein by reference in their entirety.

Replacement of an amido linkage in the peptide with a --CH₂ -sulfonamide linkage can be achieved by reducing the carboxyl (--COOH) group of a suitably protected amino acid to the --CH₂ OH group and the hydroxyl group is then converted to a suitable leaving group such as a tosyl group by conventional methods. Reaction of the tosylated derivative with, for example, thioacetic acid followed by hydrolysis and oxidative chlorination will provide for the --CH₂ --S(O)₂ Cl functional group which replaces the carboxyl group of the otherwise suitably protected amino acid. Use of this suitably protected amino acid analogue in peptide synthesis provides for inclusion of an --CH₂ S(O)₂ NR-- linkage which replaces the amido linkage in the peptide thereby providing a peptide mimetic. For a more complete description on the conversion of the carboxyl group of the amino acid to a --CH₂ S(O)₂ Cl group, see, for example, Chemistry & Biochemistry of Amino Acids, Peptides and Proteins, Boris Weinstein (ed.), Vol. 7, pp. 267-357, Marcel Dekker, Inc., New York (1983) which is incorporated herein by reference.

Replacement of an amido linkage in the peptide with a urea linkage can be achieved in the manner set forth in U.S. patent application Ser. No. 08/147,805 which application is incorporated herein by reference in its entirety.

Secondary amine linkages wherein a --CH₂ NH-- linkage replaces the amido linkage in the peptide can be prepared by employing, for example, a suitably protected dipeptide analogue wherein the carbonyl bond of the amido linkage has been reduced to a CH₂ group by conventional methods. For example, in the case of diglycine, reduction of the amide to the amine will yield after deprotection H₂ NCH₂ CH₂ NHCH₂ COOH which is then used in N-protected form in the next coupling reaction. The preparation of such analogues by reduction of the carbonyl group of the amido linkage in the dipeptide is well known in the art.

The suitably protected amino acid analogue is employed in the conventional peptide synthesis in the same manner as would the corresponding amino acid. For example, typically about 3 equivalents of the protected amino acid analogue are employed in this reaction. An inert organic diluent such as methylene chloride or DMF is employed and, when an acid is generated as a reaction by-product, the reaction solvent will typically contain an excess amount of a tertiary amine to scavenge the acid generated during the reaction. One particularly preferred tertiary amine is diisopropylethylamine which is typically employed in about 10 fold excess. The reaction results in incorporation into the peptide mimetic of an amino acid analogue having a non-peptidyl linkage. Such substitution can be repeated as desired such that from zero to all of the amido bonds in the peptide have been replaced by non-amido bonds.

One can also cyclize the peptides of the invention, or incorporate a desamino or descarboxy residue at the terminii of the peptide, so that there is no terminal amino or carboxyl group, to decrease susceptibility to proteases or to restrict the conformation of the peptide. C-terminal functional groups of the compounds of the present invention include amide, amide lower alkyl, amide di(lower alkyl), lower alkoxy, hydroxy, and carboxy, and the lower ester derivatives thereof, and the pharmaceutically acceptable salts thereof.

E. Disulfide Bond Formation

The compounds of the present invention may exist in a cyclized form with an intramolecular disulfide bond between the thiol groups of the cysteines. Alternatively, an intermolecular disulfide bond between the thiol groups of the cysteines can be produced to yield a dimeric (or higher oligomeric) compound. One or more of the cysteine residues may also be substituted with a homocysteine. These intramolecular or intermolecular disulfide derivatives can be represented schematically as shown below: ##STR1## wherein m and n are independently 1 or 2.

Other embodiments of this invention provide for analogs of these disulfide derivatives in which one of the sulfurs has been replaced by a CH₂ group or other isostere for sulfur. These analogs can be made via an intramolecular or intermolecular displacement, using methods known in the art as shown below: ##STR2## wherein p is 1 or 2. One of skill in the art will readily appreciate that this displacement can also occur using other homologs of the a-amino-g-butyric acid derivative shown above and homocysteine.

Alternatively, the amino-terminus of the peptide can be capped with an alpha-substituted acetic acid, wherein the alpha substituent is a leaving group, such as an a-haloacetic acid, for example, a-chloroacetic acid, a-bromoacetic acid, or a-iodoacetic acid. The compounds of the present invention can be cyclized or dimerized via displacement of the leaving group by the sulfur of the cysteine or homocysteine residue. See, e.g., Barker et al. (1992) J. Med. Chem. 35:2040-2048 and Or et al. (1991) J. Org. Chem. 56:3146-3149, each of which is incorporated herein by reference.

V. Utility

The compounds of the invention are useful in vitro as unique tools for understanding the biological role of TPO, including the evaluation of the many factors thought to influence, and be influenced by, the production of TPO and the receptor binding process. The present compounds are also useful in the development of other compounds that bind to and activate the TPO-R, because the present compounds provide important information on the relationship between structure and activity that should facilitate such development.

The compounds are also useful as competitive binders in assays to screen for new TPO receptor agonists. In such assay embodiments, the compounds of the invention can be used without modification or can be modified in a variety of ways; for example, by labeling, such as covalently or non-covalently joining a moiety which directly or indirectly provides a detectable signal. In any of these assays, the materials thereto can be labeled either directly or indirectly. Possibilities for direct labeling include label groups such as: radiolabels such as ¹²⁵ I, enzymes (U.S. Pat. No. 3,645,090) such as peroxidase and alkaline phosphatase, and fluorescent labels (U.S. Pat. No. 3,940,475) capable of monitoring the change in fluorescence intensity, wavelength shift, or fluorescence polarization. Possibilities for indirect labeling include biotinylation of one constituent followed by binding to avidin coupled to one of the above label groups. The compounds may also include spacers or Sinkers in cases where the compounds are to be attached to a solid support.

Moreover, based on their ability to bind to the TPO receptor, the peptides of the present invention can be used as reagents for detecting TPO receptors on living cells, fixed cells, in biological fluids, in tissue homogenates, in purified, natural biological materials, etc. For example, by labelling such peptides, one can identify cells having TPO-R on their surfaces. In addition, based on their ability to bind the TPO receptor, the peptides of the present invention can be used in in situ staining, FACS (fluorescence-activated cell sorting), Western blotting, ELISA, etc. In addition, based on their ability to bind to the TPO receptor, the peptides of the present invention can be used in receptor purification, or in purifying cells expressing TPO receptors on the cell surface (or inside permeabilized cells).

The compounds of the present invention can also be utilized as commercial reagents for various medical research and diagnostic uses. Such uses include but are not limited to: (1) use as a calibration standard for quantitating the activities of candidate TPO agonists in a variety of functional assays; (2) use to maintain the proliferation and growth of TPO-dependent cell lines; (3) use in structural analysis of the TPO-receptor through co-crystallization; (4) use to investigate the mechanism of TPO signal transduction/receptor activation; and (5) other research and diagnostic applications wherein the TPO-receptor is preferably activated or such activation is conveniently calibrated against a known quantity of a TPO agonist, and the like.

The compounds of the present invention can be used for the in vitro expansion of megakaryocytes and their committed progenitors, both in conjunction with additional cytokines or on their own. See, e.g., DiGiusto et al. PCT Publication No. 95/05843, which is incorporated herein by reference. Chemotherapy and radiation therapies cause thrombocytopenia by killing the rapidly dividing, more mature population of megakaryocytes. However, these therapeutic treatments can also reduce the number and viability of the immature, less mitotically active megakaryocyte precursor cells. Thus, amelioration of the thrombocytopenia by the compounds of the present invention can be hastened by using patients post chemotherapy or radiation therapy with a population of his or her own cells enriched for megakyocytes and immature precursors by in vitro culture.

The compounds of the invention can also be administered to warm blooded animals, including humans, to activate the TPO-R in vivo. Thus, the present invention encompasses methods for therapeutic treatment of TPO related disorders that comprise administering a compound of the invention in amounts sufficient to mimic the effect of TPO on TPOR in vivo. For example, the peptides and compounds of the invention can be administered to treat a variety of hematological disorders, including but not limited to platelet disorders and thrombocytopenia, particularly when associated with bone marrow transfusions, radiation therapy, and chemotherapy.

The activity of the compounds of the present invention can be evaluated either in vitro or in vivo in one of the numerous models described in McDonald (1992) Am. J. of Pediatric Hematology/Oncology 14:8-21, which is incorporated herein by reference.

According to one embodiment, the compositions of the present invention are useful for treating thrombocytopenia associated with bone marrow transfusions, radiation therapy, or chemotherapy. The compounds typically will be administered prophylactically prior to chemotherapy, radiation therapy, or bone marrow transplant or after such exposure.

Accordingly, the present invention also provides pharmaceutical compositions comprising, as an active ingredient, at least one of the peptides or peptide mimetics of the invention in association with a pharmaceutical carrier or diluent. The compounds of this invention can be administered by oral, pulmonary, parental (intramuscular, intraperitoneal, intravenous (IV) or subcutaneous injection), inhalation (via a fine powder formulation). transdermal, nasal, vaginal, rectal, or sublingual routes of administration and can be formulated in dosage forms appropriate for each route of administration. See, e.g., Bernstein et al. PCT Patent Publication No. WO 93/25221; Pitt et al. PCT Patent Publication No. WO 94/17784; and Pitt et al. European Patent Application 613,683, each of which is incorporated herein by reference.

Solid dosage forms for oral administration include capsules, tablets, pills, powders, and granules. In such solid dosage forms, the active compound is admixed with at least one inert pharmaceutically acceptable carrier such as sucrose, lactose, or starch. Such dosage forms can also comprise, as is normal practice, additional substances other than inert diluents, e.g., lubricating agents such as magnesium stearate. In the case of capsules, tablets, and pills, the dosage forms may also comprise buffering agents. Tablets and pills can additionally be prepared with enteric coatings.

Liquid dosage forms for oral administration include pharmaceutically acceptable emulsions, solutions, suspensions, syrups, with the elixers containing inert diluents commonly used in the art, such as water. Besides such inert diluents, compositions can also include adjuvants, such as wetting agents, emulsifying and suspending agents, and sweetening, flavoring, and perfuming agents.

Preparations according to this invention for parental administration include sterile aqueous or non-aqueous solutions, suspensions, or emulsions. Examples of non-aqueous solvents or vehicles are propylene glycol, polyethylene glycol, vegetable oils, such as olive oil and corn oil, gelatin, and injectable organic esters such as ethyl oleate. Such dosage forms may also contain adjuvants such as preserving, wetting, emulsifying, and dispersing agents. They may be sterilized by, for example, filtration through a bacteria retaining filter, by incorporating sterilizing agents into the compositions, by irradiating the compositions, or by heating the compositions. They can also be manufactured using sterile water, or some other sterile injectable medium, immediately before use.

Compositions for rectal or vaginal administration are preferably suppositories which may contain, in addition to the active substance, excipients such as cocoa butter or a suppository wax. Compositions for nasal or sublingual administration are also prepared with standard excipients well known in the art.

The compositions containing the compounds can be administered for prophylactic and/or therapeutic treatments. In therapeutic applications, compositions are administered to a patient already suffering from a disease, as described above, in an amount sufficient to cure or at least partially arrest the symptoms of the disease and its complications. An amount adequate to accomplish this is defined as "therapeutically effective dose." Amounts effective for this use will depend on the severity of the disease and the weight and general state of the patient.

In prophylactic applications, compositions containing the compounds of the invention are administered to a patient susceptible to or otherwise at risk of a particular disease. Such an amount is defined to be a "prophylactically effective dose." In this use, the precise amounts again depend on the patient's state of health and weight.

The quantities of the TPO agonist necessary for effective therapy will depend upon many different factors, including means of administration, target site, physiological state of the patient, and other medicants administered. Thus, treatment dosages should be titrated to optimize safety and efficacy. Typically, dosages used in vitro may provide useful guidance in the amounts useful for in situ administration of these reagents. Animal testing of effective doses for treatment of particular disorders will provide further predictive indication of human dosage. Various considerations are described, e.g., in Gilman et al. (eds), (1990) Goodman and Gilman's: The Pharmacological Basis of Therapeutics, 8th ed., Pergamon Press; and Remington's Pharmaceutical Sciences, (1985) 7th ed., Mack Publishing Co., Easton, Pa.; each of which is hereby incorporated by reference.

The peptides and peptide mimetics of this invention are effective in treating TPO mediated conditions when administered at a dosage range of from about 0.001 mg to about 10 mg/kg of body weight of the mammal per day. The specific dose employed is regulated by the particular condition being treated, the route of administration as well as by the judgement of the attending clinician depending upon factors such as the severity of the condition, the age and general condition of the patient, and the like.

Although only preferred embodiments of the invention are specifically described above, it will be appreciated that modifications and variations of the invention are possible without departing from the spirit and intended scope of the invention.

EXAMPLE 1 Solid Phase Peptide Synthesis

Various peptides of the invention were synthesized using the Merrifield solid phase synthesis techniques (see Steward and Young, Solid Phase Peptide Synthesis, 2d. edition (Pierce Chemical, Rockford, Ill. (1984) and Merrifield (1963) J. Am. Chem. Soc. 85:2149) on a Milligen/Biosearch 9600 automated instrument or an Applied Biosystems Inc. Model 431A peptide synthesizer. The peptides were assembled using standard protocols of the Applied Biosystems Inc. System Software version 1.01. Each coupling was performed for one-two hours with BOP (benzotriazolyl N-oxtrisdimethylaminophosphonium hexafluorophosphate) and HOBt (1-hydroxybenzotriazole).

The resin used was HMP resin or PAL Milligen/Biosearch), which is a cross-linked polystyrene resin with 5-(4'-Fmoc-aminomethyl-3,5'-dimethyoxyphenoxy)valeric acid as a liner. Use of PAL resin results in a carboxyl terminal amide functionality upon cleavage of the peptide from the resin. Upon cleavage, the HMP resin produces a carboxylic acid moiety at the C-terminus of the final product. Most reagents, resins, and protected amino acids (free or on the resin) were purchased from Millipore or Applied Biosystems Inc.

The Fmoc group was used for amino protection during the coupling procedure. Primary amine protection on amino acids was achieved with Fmoc and side chain protection groups were t-butyl for serine, tyrosine, asparagine, glutamic acid, and threonine; trityl for glutamine; Pmc (2,2,5,7,8-pentamethylchroma sulfonate) for arginine; N-t-butyloxycarbonyl for tryptophan; N-trityl for histidine and glutamine; and S-trityl for cysteine.

Removal of the peptides from the resin and simultaneous deprotection of the side chain functions were achieved by treatment with reagent K or slight modifications of it. Alternatively, in the synthesis of those peptides, with an amidated carboxyl terminus, the fully assembled peptide was cleaved with a mixture of 90% trifluoroacetic acid, 5% ethanedithiol, and 5% water, initially at 4° C., and gradually increasing to room temperature. The deprotected peptides were precipitated with diethyl ether. In all cases, purification was by preparative, reverse-phase, high performance liquid chromatography on a C₁₈ bonded silica gel column with a gradient of acetonitrile/water in 0.1% trifluoroacetic acid. The homogeneous peptides were characterized by Fast Atom Bombardment mass spectrometry or electrospray mass spectrometry and amino acid analysis when applicable.

EXAMPLE 2 Bioassays

Bioactivity of the peptides can be measured using a thrombopoietin dependent cell proliferation assay. Murine IL-3 dependent Ba/F3 cells were transfected with full length human TPO-R. In the absence of IL-3 (WEHI-3 conditioned media), these cells are dependent on TPO for proliferation. The parental, untransfected cell line does not respond to human TPO, but remains IL-3 dependent.

Bioassays have been performed on both of the above cell lines using synthetic peptides derived from library screening. The cells were grown in complete RPMI-10 media, containing 10% WEHI-3 conditioned media, then washed twice in PBS, resuspended in media which lacked WEHI-3 conditioned media, and added to wells containing dilutions of peptide or TPO at 2×10⁴ cells/well. The cells were incubated for 48 hours at 37° C. in a humidified 5% CO₂ atmosphere and metabolic activity was assayed by the reduction of MTT to formazan, with absorbance at 570 nM measured on an ELISA plate reader. The peptides tested stimulated proliferation of TPO-R transfected Ba/F3 cells in a dose dependent manner as shown in FIG. 1. These peptides have no effect on the parental cell line.

EXAMPLE 3 Binding Affinity

Binding affinities of chemically synthesized peptides for TPO-R were measured in a competition binding assay. The wells of a microtiter plate were coated with 1 mg streptavidin, blocked with PBS/1% BSA, followed by 50 ng of biotinylated anti-receptor immobilizing antibody (Ab179). The wells were then treated with a 1:10 dilution of soluble TPO-R harvest. Various concentrations of peptide or peptide mimetic were mixed with a constant amount of a truncated form of TPO consisting of residues 1-156 fused to the C-terminus of maltose binding protein (MBP-TPO₁₅₆). The peptide MBP-TPO₁₅₆ mixtures were added to the TPO-R coated wells, incubated for 2 hours at 4° C. and then washed with PBS. The amount of MBP-TPO₁₅₆ that was bound at equilibrium was measured by adding a rabbit anti-sera directed against MBP, followed by alkaline phosphatase conjugated goat anti-rabbit IgG. The amount of alkaline phosphatase in each well was then determined using standard methods.

The assay is conducted over a range of peptide concentrations and the results are graphed such that the y axis represents the amount of bound MBP-TPO₁₅₆ and the x axis represents the concentration of peptide or peptide mimetic. One can then determine the concentration at which the peptide or peptide mimetic will reduce by 50% (IC₅₀) the amount of MBP-TPO₁₅₆ bound to immobilized TPO-R. The dissociation constant (Kd) for the peptide should be similar to the measured IC₅₀ using the assay conditions described above.

EXAMPLE 4 "Peptides on Plasmids"

The pJS142 vector is used for library construction and is shown in FIG. 2. Three oligonucleotide sequences are needed for library construction: ON-829 (5' ACC ACC TCC GG); ON-830 (5' TTA CTT AGT TA) and a library specific oligonucleotide of interest (5' GA GGT GGT {NNK}_(n) TAA CTA AGT AAA GC), where {NNK}_(n) denotes a random region of the desired length and sequence. The oligonucleotides can be 5' phosphorylated chemically during synthesis or after purification with polynucleotide kinase. They are then annealed at a 1:1:1 molar ratio and ligated to the vector.

The strain of E. coli which is preferably used for panning has the genotype: Δ(srl-recA) endA1 nupG lon-11 sulA1 hsdR17 Δ(ompT-fepC)266 ΔclpA319::kan ΔlacI lac ZU118 which can be prepared from an E. coli strain from the E. coli Genetic Stock Center at Yale University (E. coli b/r, stock center designation CGSC:6573) with genotype lon-11 sulA1. The above E. coli strain is prepared for use in electroporation as described by Dower et al. (1988) Nucleic Acids Res. 16:6127, except that 10% glycerol is used for all wash steps. The cells are tested for efficiency using 1 pg of a Bluescript plasmid (Stratagene). These cells are used for growth of the original library and for amplification of the enriched population after each round of panning.

Peptides on plasmids are released from cells for panning by gentle enzymatic digestion of the cell wall using lysozyme. After pelleting of the cell debris, the crude lysate can be used directly on most receptors. If some additional purification of the plasmid complexes is needed, a gel filtration column can be used to remove many of the low molecular weight contaminants in the crude lysate.

Panning is carried out in a buffer (HEKL) of a lower salt concentration than most physiological buffers. The panning can be conducted in microtiter wells with a receptor immobilized on a nonblocking monoclonal antibody (MAb) or by panning on beads or on columns. More specifically, in the first round of panning, 24 wells, each coated with receptor, can be used. For the second round, six wells coated with receptor (PAN sample) and 6 wells without receptor (NC sample) are typically used. Comparison of the number of plasmids in these two samples can give an indication of whether receptor specific clones are being enriched by panning. "Enrichment" is defined as the ratio of PAN transformants to those recovered from the NC sample. Enrichment of 10 fold is usually an indication that receptor specific clones are present.

In later rounds of panning, it is useful to reduce the input of lysate into the wells to lower nonspecific background binding of the plasmid complexes. In round 2, usually 100 ml of lysate per well is used. In round 3, 100 ml of lysate per well diluted with 1/10 in HEKL/BSA is used. For further rounds of panning, typically an input of plasmid transforming units of at least 1000 fold above the estimated remaining diversity is used.

The binding properties of the peptides encoded by individual clones are typically examined after 3, 4, or 5 rounds of panning, depending on the enrichment numbers observed. Typically, an ELISA that detects receptor specific binding by LacI-peptide fusion proteins is used. LacI is normally a tetramer and the minimum functional DNA binding species is a dimer. The peptides are thus displayed multivalently on the fusion protein. Assuming that a sufficient density of receptor can be immobilized in wells, the peptides fused to LacI will bind to the surface in a cooperative, multivalent fashion. This cooperative binding permits the detection of binding events of low intrinsic affinity. The sensitivity of this assay is an advantage in that initial hits of low affinity can be easily identified, but is a disadvantage in that the signal in the ELISA is not correlated with the intrinsic affinity of the peptides. Fusion of the peptides to maltose binding protein (MBP) as described below permits testing in an ELISA format where signal strength is better correlated with affinity.

See FIG. 3.

DNA from clones of interest can be prepared in double stranded form using any standard miniprep procedure. The coding sequences of interesting single clones or populations of clones can be transferred to vectors that fuse those sequences in frame with the gene encoding MBP, a protein that generally occurs as a monomer in solution. The cloning of a library into pJS142 creates a BspEI restriction site near the beginning of the random coding region of the library. Digestion with BspEI and ScaI allows the purification of a .sup.˜ 900 bp DNA fragment that can be subcloned into one of two vectors, pELM3 (cytoplasmic) or pELM15 (periplasmic), which are simple modifications of the pMALc2 and pMALp2 vectors, respectively, available commercially from New England Biolabs. See FIGS. 3A-B. Digestion of pELM3 and pELM15 with AgeI and ScaI allows efficient cloning of the BspEI--ScaI fragment from the pJS142 library. The BspEI and AgeI ends are compatible for ligation. In addition, correct ligation of the ScaI sites is essential to recreate a functional bla (Amp resistance) gene, thus lowering the level of background clones from undesired ligation events. Expression of the tac promoter-driven MBP-peptide fusions can then be induced with IPTG.

Lysates for the LacI or MBP ELISAs are prepared from individual clones by lysing cells using lysozyme and removing insoluble cell debris by centrifugation. The lysates are then added to wells containing immobilized receptor and to control wells without receptor. Binding by the LacI or MBP peptide fusions is detected by incubation with a rabbit polyclonal antiserum directed against either LacI or MBP followed by incubation with alkaline phosphatase labeled goat anti rabbit second antibody. The bound alkaline phosphatase is detected with p-nitrophenyl phosphate chromagenic substrate.

EXAMPLE 5 "Headpiece Dimer" System

A variant of the Lacd peptides-on-plasmids technique utilizes a DNA binding protein called "headpiece dimer". DNA binding by the E. coli lac repressor is mediated by the approximately 60 amino acid "headpiece" domain. The dimer of the headpiece domains that binds to the lac operator is normally formed by association of the much larger approximately 300 amino acid C-terminal domain. The "headpiece dimer" system utilizes headpiece dimer molecules containing two headpieces connected via short peptide linker. These proteins bind DNA with sufficient stability to allow association of a peptide epitope displayed at the C-terminus of the headpiece dimer with the plasmid encoding that peptide.

The random peptides are fused to the C-terminus of the headpiece dimer, which binds to the plasmid that encoded it to make a peptide-headpiece dimer-plasmid complex that can be screened by panning. The headpiece dimer peptides-on-plasmids system allows greater selectivity for high affinity ligands than the LacI system. Thus, the headpiece dimer system is useful for making mutagenesis libraries based on initial low-affinity hits, and selecting higher affinity variants of those initial sequences.

The libraries are constructed as with pep tides on plasmids using headpiece dimer vector pCMG14 (see FIG. 4). The presence of the lac operator is not required for plasmid binding by the headpiece dimer protein. The libraries were introduced into bacterial strain comprising E. coli (lon-11 su1A1 hsdR17 (ompT-fepC) ΔclpA319::kan ΔlacI lac ZU118 Δ(srl-recA) 306::Tn10 .and amplified under conditions of basal (A) promoter induction. Panning of headpiece dimer libraries is carried out by similar procedures to those used for LacI libraries, except that HEK buffer is used instead of HEKL buffer and elution of plasmids from the wells is performed with aqueous phenol instead of with IPTG. Sequences from headpiece dimer panning are often characterized after transfer to the MBP vector so that they can be tested in the affinity sensitive MBP ELISA and also so that populations of clones can be screened by colony lifts with labeled receptor.

The disclosures in this application of all articles and references, including patent documents, are incorporated herein by reference.

    __________________________________________________________________________     #             SEQUENCE LISTING     - (1) GENERAL INFORMATION:     -    (iii) NUMBER OF SEQUENCES: 58     - (2) INFORMATION FOR SEQ ID NO:1:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 9 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (ix) FEATURE:               (A) NAME/KEY: Peptide               (B) LOCATION: 1..9     #/note= "Sequence reads: VRX1QX2XXXX3.                    Peptide i - #n which X1=D/E; X2=I/V;                    X3=F,H,L, - #M,T,V,W or Y; and each X residue is                    independentl - #y selected from the 20 genetically coded     #acids, their stereoisomeric D-amino acids and     #amino acids." non-natural     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:1:     -      Val Arg Xaa Gln Xaa Xaa Xaa Xaa - # Xaa     #   51     - (2) INFORMATION FOR SEQ ID NO:2:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 9 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (ix) FEATURE:               (A) NAME/KEY: Peptide               (B) LOCATION: 1..9     #/note= "Sequence reads:RMATION:                    VRX1QX2X4X5X - #6X3.  Peptide in which X1=D/E;     #X3=F,H,L,M,T,V,W or Y; X4=A,C,F,H,I,L.M.Q,S,T,V,W     #X5=A,C,G,I,K,L,M,Q,R,S,W or V; and                    X6=A,C,F, - #H,L,R,S,W, or Y."     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:2:     -      Val Arg Xaa Gln Xaa Xaa Xaa Xaa - # Xaa     #   51     - (2) INFORMATION FOR SEQ ID NO:3:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 11 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (ix) FEATURE:               (A) NAME/KEY: Peptide               (B) LOCATION: 1..11     #/note= "Sequence reads:RMATION:                    XXVRX1QX2X4X - #6X6X3.  Peptide in which each                    X residue - # is independently selected from any of the     20     #coded L-amino acids, their stereoisomeric                    D-amino a - #cids, and non-natural amino acids."     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:3:     -      Xaa Xaa Val Arg Xaa Gln Xaa Xaa - # Xaa Xaa Xaa     #       10     - (2) INFORMATION FOR SEQ ID NO:4:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 11 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (ix) FEATURE:               (A) NAME/KEY: Peptide               (B) LOCATION: 1..11     #/note= "Sequence reads:RMATION:                    X7X8VRX1QX2X - #4X5X6X3.  Peptide in which                    X7=A,E,G, - #I,L,N,Q,P,R,S,T,V or Y; X8=D,E,G,K,R,S,T or     Y;     #X2=I/V; X3=F,H,L,M,T,V,W or Y;                    X4=A,C,F, - #H,I,L,M,Q,S,T,V,W or Y; X5=A,C,G,I,K,L,M,Q,R,S     ,W     #X6=A,C,F,H,L,R,S,W or Y."     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:4:     -      Xaa Xaa Val Arg Xaa Gln Xaa Xaa - # Xaa Xaa Xaa     #   10     - (2) INFORMATION FOR SEQ ID NO:5:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 14 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:5:     -      Gln Asp Pro Arg Gly Val Arg Glu - # Gln Val Ile Ala Arg Leu     #   10     - (2) INFORMATION FOR SEQ ID NO:6:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 10 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:6:     -      Gly Arg Val Arg Asp Gln Val Ala - # Gly Trp     #   10     - (2) INFORMATION FOR SEQ ID NO:7:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 10 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:7:     -      Gly Arg Val Lys Asp Gln Ile Ala - # Gln Leu     #   10     - (2) INFORMATION FOR SEQ ID NO:8:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 10 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:8:     -      Gly Val Arg Asp Gln Val Ser Trp - # Ala Leu     #   10     - (2) INFORMATION FOR SEQ ID NO:9:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 10 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:9:     -      Glu Ser Val Arg Glu Gln Val Met - # Lys Tyr     #   10     - (2) INFORMATION FOR SEQ ID NO:10:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 10 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:10:     -      Ser Val Arg Ser Gln Ile Ser Ala - # Ser Leu     #   10     - (2) INFORMATION FOR SEQ ID NO:11:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 10 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:11:     -      Gly Val Arg Asp Gln Val Ser Trp - # Ala Leu     #   10     - (2) INFORMATION FOR SEQ ID NO:12:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 10 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:12:     -      Gly Val Arg Glu Thr Val Tyr Arg - # His Met     #   10     - (2) INFORMATION FOR SEQ ID NO:13:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 11 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:13:     -      Gly Val Arg Glu Val Ile Val Met - # His Met Leu     #   10     - (2) INFORMATION FOR SEQ ID NO:14:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 11 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:14:     -      Ser Asp Leu Arg Asp Cys Ile Leu - # Met Leu His     #   10     - (2) INFORMATION FOR SEQ ID NO:15:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 11 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:15:     -      Ala Gly Val Arg Asp Gln Ile Leu - # Ile Trp Leu     #   10     - (2) INFORMATION FOR SEQ ID NO:16:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 11 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:16:     -      Gly Arg Val Arg Asp Gln Ile Met - # Leu Ser Leu     #   10     - (2) INFORMATION FOR SEQ ID NO:17:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 11 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:17:     -      Gly Val Arg Gln Gln Val Phe Ala - # Tyr Leu Val     #   10     - (2) INFORMATION FOR SEQ ID NO:18:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 11 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:18:     -      Leu Ser Val Arg Asp Gln Val Met - # Ala Trp Leu     #   10     - (2) INFORMATION FOR SEQ ID NO:19:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 11 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:19:     -      Gly Lys Val Arg Asp Gln Val Tyr - # Leu His Leu     #   10     - (2) INFORMATION FOR SEQ ID NO:20:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 11 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:20:     -      Leu Ser Val Arg Glu Gln Val Met - # Ala Trp Leu     #   10     - (2) INFORMATION FOR SEQ ID NO:21:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 11 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:21:     -      Gly Arg Val Arg Asp Gln Ile Trp - # Ala Ala Leu     #   10     - (2) INFORMATION FOR SEQ ID NO:22:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 11 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:22:     -      Ser Ser Val Arg Glu Gln Val Cys - # Leu Phe Phe     #   10     - (2) INFORMATION FOR SEQ ID NO:23:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 11 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:23:     -      Gly Arg Val Arg Glu Gln Val Ala - # Tyr Cys Leu     #   10     - (2) INFORMATION FOR SEQ ID NO:24:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 15 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:24:     -      Thr Gly Leu Arg Ser Val Arg Asp - # Gln Val Cys Ala His Phe Ala     #   15     - (2) INFORMATION FOR SEQ ID NO:25:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 15 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:25:     -      Tyr Ser Ser Gly Ser Val Arg Glu - # Gln Val Met Ala Phe Leu Ala     #   15     - (2) INFORMATION FOR SEQ ID NO:26:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 15 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:26:     -      Val Gly Ser Asn Ser Val Arg Asp - # Gln Val Thr Ala Trp Leu Leu     #   15     - (2) INFORMATION FOR SEQ ID NO:27:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 15 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:27:     -      Ala Thr Gly Asn Arg Val Arg Glu - # Gln Val Phe Trp Tyr Phe Leu     #   15     - (2) INFORMATION FOR SEQ ID NO:28:     -      (i) SEQUENCE CHARACTERISTICS:               (A) LENGTH: 2 base p - #airs               (B) TYPE: nucleic acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: nucleic acid     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:28:     #               2     - (2) INFORMATION FOR SEQ ID NO:29:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 15 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:29:     -      Tyr Val Arg Glu Ser Val Arg Asp - # Gln Val Trp Lys Phe Tyr Cys     #   15     - (2) INFORMATION FOR SEQ ID NO:30:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 15 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:30:     -      Ser Gly Ser Leu Ser Val Arg Glu - # Gln Val His Leu Trp Leu Ala     #   15     - (2) INFORMATION FOR SEQ ID NO:31:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 15 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:31:     -      Ser Glu Arg Gln Ser Val Arg Glu - # Gln Val Ser Gln Trp Leu Leu     #   15     - (2) INFORMATION FOR SEQ ID NO:32:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 15 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:32:     -      Gln Val Arg Gly Ser Val Arg Glu - # Gln Val Met Ile Trp Leu Ser     #   15     - (2) INFORMATION FOR SEQ ID NO:33:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 15 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:33:     -      Gly Ser Asn Gln Ser Val Arg Glu - # Gln Val Trp Ala Tyr Val Met     #   15     - (2) INFORMATION FOR SEQ ID NO:34:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 15 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:34:     -      Thr Gly Leu Arg Ser Val Arg Asp - # Gln Val Cys Ala His Phe Ala     #   15     - (2) INFORMATION FOR SEQ ID NO:35:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 15 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:35:     -      Arg Pro Trp Thr Ser Val Arg Glu - # Gln Val Tyr Leu His Leu Ile     #   15     - (2) INFORMATION FOR SEQ ID NO:36:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 14 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:36:     -      Tyr Ser Arg Gly Ser Val Arg Asp - # Val Phe Ser Trp Leu His     #   10     - (2) INFORMATION FOR SEQ ID NO:37:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 15 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:37:     -      Asp Met Val Gln Ser Val Arg Glu - # Gln Val Ala Leu Phe Leu Ala     #   15     - (2) INFORMATION FOR SEQ ID NO:38:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 15 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:38:     -      Glu Thr Arg Thr Ser Val Arg Glu - # Gln Val Ala Ala Phe Leu Ala     #   15     - (2) INFORMATION FOR SEQ ID NO:39:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 15 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:39:     -      Arg Ser Ser Val Ser Val Arg Asp - # Gln Val Leu Ala Phe Leu Ser     #   15     - (2) INFORMATION FOR SEQ ID NO:40:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 14 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:40:     -      Asn Pro Gly Leu Ser Val Arg Asp - # Val Phe Leu His Phe Phe     #   10     - (2) INFORMATION FOR SEQ ID NO:41:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 15 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:41:     -      Trp Thr Arg Ile Ser Val Arg Asp - # Gln Val Trp Ala Phe Tyr His     #   15     - (2) INFORMATION FOR SEQ ID NO:42:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 15 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:42:     -      Arg Pro Thr Ser Arg Val Arg Glu - # Gln Val Met Leu Trp Leu Ser     #   15     - (2) INFORMATION FOR SEQ ID NO:43:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 15 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:43:     -      Ser Arg Gly Ala Ser Val Arg Asp - # Gln Val Val Leu His Leu Cys     #   15     - (2) INFORMATION FOR SEQ ID NO:44:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 15 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:44:     -      Ala Gly Val Pro Ser Val Arg Glu - # Gln Val Val Ala Arg Leu Cys     #   15     - (2) INFORMATION FOR SEQ ID NO:45:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 15 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:45:     -      Gly Glu Thr Arg Ser Val Arg Asp - # Gln Val Ile Ala Arg Leu Cys     #   15     - (2) INFORMATION FOR SEQ ID NO:46:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 15 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:46:     -      Ser Gly Arg Gln Ser Val Arg Asp - # Gln Val Leu Gly Cys Thr Trp     #   15     - (2) INFORMATION FOR SEQ ID NO:47:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 14 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:47:     -      Ser His Met Ser Val Arg Glu Gln - # Val Gln Tyr Tyr Leu Leu     #   10     - (2) INFORMATION FOR SEQ ID NO:48:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 15 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:48:     -      Ser Ala Cys Cys Arg Val Arg Asp - # Gln Val Phe Trp Phe Trp His     #   15     - (2) INFORMATION FOR SEQ ID NO:49:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 15 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:49:     -      Ser Gln Leu Ala Thr Val Arg Glu - # Gln Val Tyr Ala Phe Trp Cys     #   15     - (2) INFORMATION FOR SEQ ID NO:50:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 15 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:50:     -      Ala Lys Asn Leu Ser Val Arg Glu - # Gln Val Met Leu His Leu Cys     #   15     - (2) INFORMATION FOR SEQ ID NO:51:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 15 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:51:     -      Ser Gly Pro Asp Ser Val Arg Glu - # Gln Val Ile Ser Arg Leu Cys     #   15     - (2) INFORMATION FOR SEQ ID NO:52:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 15 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:52:     -      Thr Glu Arg Leu Ser Val Arg Glu - # Gln Val Leu Cys Trp Val Thr     #   15     - (2) INFORMATION FOR SEQ ID NO:53:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 15 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:53:     -      Trp Thr Arg Thr Ser Val Arg Asp - # Gln Val Ala Leu Trp Leu His     #   15     - (2) INFORMATION FOR SEQ ID NO:54:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 15 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:54:     -      Trp Thr Arg Thr Ser Val Arg Asp - # Gln Val Ala Leu Trp Leu His     #   15     - (2) INFORMATION FOR SEQ ID NO:55:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 15 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:55:     -      Glu Gly Xaa Asn Tyr Val Arg Glu - # Gln Val Cys Ala Phe Phe Tyr     #   15     - (2) INFORMATION FOR SEQ ID NO:56:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 15 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:56:     -      Ala Thr Thr Arg Ser Val Arg Glu - # Gln Val Tyr Leu Phe Leu Ser     #   15     - (2) INFORMATION FOR SEQ ID NO:57:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 15 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:57:     -      Arg Val Gly Leu Ser Val Arg Asp - # Gln Val Tyr Ala Trp Leu Ala     #   15     - (2) INFORMATION FOR SEQ ID NO:58:     -      (i) SEQUENCE CHARACTERISTICS:     #acids    (A) LENGTH: 13 amino               (B) TYPE: amino acid               (C) STRANDEDNESS: single               (D) TOPOLOGY: linear     -     (ii) MOLECULE TYPE: peptide     -     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:58:     -      Gly Arg Val Arg Asp Gln Ile Met - # Leu Ser Leu Gly Gly     #   10     __________________________________________________________________________ 

What is claimed is:
 1. A compound that binds to thrombopoietin receptor, said compound having:(1) a molecular weight of less than about 5000 daltons, and (2) a binding affinity to thrombopoietin receptor as expressed by an IC₅₀ of no more than about 100 mm.
 2. The compound of claim 1, wherein said compound is a peptide, and,wherein from zero to all of the --C(O)NH-- linkages of the peptide have been replaced by a linkage selected from the group consisting of a --CH₂ OC(O)NR-- linkage; a phosphonate linkage; a --CH₂ S(O)₂ NR-- linkage; a --CH₂ NR-- linkage; and a --C(O)NR⁶ -- linkage; and a --NHC(O)NH-- linkage where R is hydrogen or lower alkyl and R⁶ is lower alkyl, further wherein the N-terminus of said peptide or peptide mimetic is selected from the group consisting of a --NRR¹ group; a --NRC(O)R group; a --NRC(O)OR group; a --NRS(O)₂ R group; a --NHC(O)NHR group; a succinimide group; a benzyloxycarbonyl-NH-- group; and a benzyloxycarbonyl-NH-- group having from 1 to 3 substituents on the phenyl ring selected from the group consisting of lower alkyl, lower alkoxy, chloro, and bromo, where R and R¹ are independently selected from the group consisting of hydrogen and lower alkyl, and still further wherein the C-terminus of said peptide or peptide mimetic has the formula --C(O)R² where R² is selected from the group consisting of hydroxy, lower alkoxy, and --NR³ R⁴ where R³ and R⁴ are independently selected from the group consisting of hydrogen and lower alkyl and where the nitrogen atom of the --NR³ R⁴ group can optionally be the amine group of the N-terminus of the peptide so as to form a cyclic peptide, and physiologically acceptable salts thereof.
 3. The compound of claim 2, wherein said compound comprises a sequence of amino acids:

    V R X.sub.1 Q X.sub.2 X X X X.sub.3 (SEQ. ID. NO.1)

where X, is D or E; X₂ is I or V; X₃ is F, H, L, M, T, V, W, or Y; and each X residue is independently selected from any of the 20 genetically coded L-amino acids.
 4. The compound of claim 3, wherein X₃ is F, L, W, or Y.
 5. The compound of claim 4, wherein said compound comprises a sequence of amino acids:

    V R X.sub.1 Q X.sub.2 X.sub.4 X.sub.5 X.sub.6 X.sub.3 (SEQ. ID. NO.2)

where X₄ is A, C, F, H, I, L, M, Q, S, T, V, W, or Y; X₅ is A, C, G, I, K, L, M, Q, R, S, W, or V; and X₆ is A, C, F, H, L, R, S, W, or Y.
 6. The compound of claim 5, wherein X₄ is A, C, F, I, L, M, S, W, or Y; X₅ is A, L, M, or W; and X₆ is F, H, R, W, or Y.
 7. The compound of claim 6, wherein said compound comprises a sequence of amino acids:

    X X V R X.sub.1 Q X.sub.2 X.sub.4 X.sub.5 X.sub.6 X.sub.3 (SEQ. ID. NO.3)

where each X residue is independently selected from any of the 20 genetically coded L-amino acids.
 8. The compound of claim 7, wherein said compound comprises a sequence of amino acids:

    X.sub.7 X.sub.8 V R X.sub.1 Q X.sub.2 X.sub.4 X.sub.5 X.sub.6 X.sub.3 (SEQ. ID. NO.4)

where X₇ is A, E, G, I, L, N, Q, P, R, S, T, V, or Y; and X₈ is D, E, G, K, R, S, T, or Y.
 9. The compound of claim 8, wherein X₇ is E, G L, R, S, or T; and X₈ is E, G, R, or S.
 10. The compound of claim 9, wherein said compound is selected from the group consisting of Q D P R G V R E Q V I A R L C (SEQ. ID. NO.5) and its disulfide linked homodimer

    QDPRGVREQVIARLC                   |     QDPRGVREQVIARLC.


11. A pharmaceutical composition comprising the compound of claim 1 in combination with a pharmaceutically acceptable carrier.
 12. A method for treating a patient suffering from a disorder that is susceptible to treatment with a thrombopoietin agonist, comprising administering to the patient, a therapeutically effective dose or amount of a compound of claim
 1. 13. The method of claim 12, wherein the compound administered to the patient is a peptide, and,wherein from zero to all of the --C(O)NH-- linkages of the peptide have been replaced by a linkage selected from the group consisting of a --CH₂ OC(O)NR-- linkage; a phosphonate linkage; a --CH₂ S(O)₂ NR-- linkage; a --CH₂ NR-- linkage; and a --C(O)NR⁶ -- age; and a --NHC(O)NH-- linkage where R is hydrogen or lower alkyl and R⁶ is lower alkyl, further wherein the N-terminus of said peptide or peptide mimetic is selected from the group consisting of a --NRR¹ group; a --NRC(O)R group; a --NRC(O)OR group; a --NRS(O)₂ R group; a --NHC(O)NHR group; a succinimide group; a benzyloxycarbonyl-NH-- group; and a benzyloxycarbonyl-NH-- group having from 1 to 3 substituents on the phenyl ring selected from the group consisting of lower alkyl, lower alkoxy, chloro, and bromo, where R and R¹ are independently selected from the group consisting of hydrogen and lower alkyl, and still further wherein the C-terminus of said peptide or peptide mimetic has the formula --C(O)R² where R² is selected from the group consisting of hydroxy, lower alkoxy, and --NR³ R⁴ where R³ and R⁴ are independently selected from the group consisting of hydrogen and lower alkyl and where the nitrogen atom of the --NR³ R⁴ group can optionally be the amine group of the N-terminus of the peptide so as to form a cyclic peptide, and physiologically acceptable salts thereof.
 14. The method of claim 13, wherein the compound administered to the patient comprises a sequence of amino acids:

    V R X.sub.1 Q X.sub.2 X X X X.sub.3 (SEQ. ID. NO.1)

where X₁ is or E; X₂ is I or V; X₃ is F, H, L, M, T, V, W, or Y; and each X residue is independently selected from any of the 20 genetically coded L-amino acids.
 15. The method of claim 14, wherein X₃ is F, L, W, or Y.
 16. The method of claim 15, wherein the compound administered to the patient comprises a sequence of amino acids:

    V R X.sub.1 Q X.sub.2 X.sub.4 X.sub.5 X.sub.6 X.sub.3 (SEQ. ID. NO.2)

where X₄ is A, C, F, H, I, L, M, Q, S, T, V, W, or Y; X₅ is A, C, G, I, K, L, M, Q, R, S, W, or V; and X₆ is A, C, F, H, L, R, S, W, or Y.
 17. The method of claim 16, wherein X₄ is A, C, F, I, L, M, S, W, or Y; X₅ is A, L, M, or W; and X₆ is F, H, R, W, or Y.
 18. The method of claim 17, wherein the compound administered to the patient comprises a sequence of amino acids:

    X X V R X.sub.1 Q X.sub.2 X.sub.4 X.sub.5 X.sub.6 X.sub.3 (SEQ. ID. NO.3)

where each X residue is independently selected from any of the 20 genetically coded L-amino acids.
 19. The method of claim 18, wherein the compound administered to the patient comprises a sequence of amino acids:

    X.sub.7 X.sub.8 V R X.sub.1 Q X.sub.2 X.sub.4 X.sub.5 X.sub.6 X.sub.3 (SEQ. ID. NO.4)

where X₇ is A, E, G, I, L, N, Q, P, R, S, T, V, or Y; and X₈ is D, E, G, K, R, S, T, or Y.
 20. The method of claim 19, wherein X₇ is E, G L, R, S, or T; and X₈ is E, G, R, or S.
 21. The method of claim 20, wherein said compound administered to the patient is selected from the group consisting of Q D P R G V R E Q V I A R L C (SEQ. ID. NO.5) and its disulfide linked homodimer

    QDPRGVREQVIARLC                   |     QDPRGVREQVIARLC.


22. The method of claim 12, wherein the disorder susceptible to treatment with a thrombopoietin agonist is selected from the group consisting of: hematological disorders and thrombocytopenia resulting from chemotherapy, radiation therapy, or bone marrow transfusions. 